A detailed account is enough to covers several books, not even accounting for the very important historiographical debate among historians on the question : with your permission, I'll mostly give a broad description of a consensus on the matter, listing possible cause and contingencies.

The general idea currently more or less agreed on, is that the fall wasn't as much civilizational (except in some peripheral areas such as Britain or Illyricum) than a relatively quick but gradual reduction of western Roman state authority during the Vth century and its replacement by new Barbarian politic and palatial structures that were essentially late Roman states on a smaller geographical scales.

The late IVth and early Vth century western Romania was arguably looking looking in a better shape than during the IIIrd century where a multi-causal crisis (called the Third Century Crisis) almost broke (economically and politically wise) the Empire with epidemics (and consequent demographic depression), climatic changes (which had a deep impact on a largely agrarian Roman economy) and military pressure (due to Sassanians much more than Barbarians, although the latter took the opportunity at hand to raid provinces).
The economic crisis, critically, created a vicious circle : less population meant a stronger reliance on slaves whom supply went down in spite of trade; which meant less taxes, less recruits and less food supplies. Barbarian raiding in the West were a real problem, but it piled up.
Along that there's the problem of hyperinflation with roman coinage being debased, silver denarius being barely worth what is used to be one century before; raises of prices, the necessity to increase wages of the army in time of critical need, etc.

Now, imperial policies since the late IIIrd tried to deal with this, and by the IVth century, the situation was unmistakably better, thanks to a certain degree of political and institutional experimentation : emperors had a more hands down approach of imperial management, the imperial court being able to move from city to city relatively quickly to address immediate issues. A system of co-imperium emerged by the late IInd century and was refined by Diocletian in order to rationalize fiscal-military logistics and execution (it didn't really worked as intended, due to the refusal of subordination and the lack of actual collegiality; but it somehow worked nevertheless); along a more direct relationship between the imperial court and the provincial elites, traditional senatorial elites of Italy and to share the access to the emperor.
The gradual adoption of Christianity as the main legal religion could be, partly, be understood as this political experimentation in the Empire, going further than being just the personal cult display of the Emperor.
On the matter of prices and inflation, Diolectian reforms managed to limit damages by reforming coinage and an indicative list of prices (it's unclear how much enforced it was) but inflation continued, if at a reduced rythm : several attempts were made by controlling the debasement of bronze, silver and gold coins under Constantinians; raising up taxes or collecting new ones were temporary situations and fiscal taxation tended to be replaced by fiscal service, notably for supply and military purposes, notables having the general fiscal responsibility. This more or less stabilized the state budget (which due to the more hand down approach of the imperial administration, had increased especially when it came to address civil and military service, both by the early Vth being increasingly fused), but did a number on the monetarisation of popular economy, and popular classes tended to be let to themselves on some aspects, or rather let to local notable support and charity (in the social sense).

This evolution is more marked in the West than in the East, in main part because european provinces suffered most of the Barbarian raids of the IIIrd century and the civil wars, while African and Asiatic provinces were relatively spared (and even, in the Vth century, underwent a renewed prosperity)

Another change was the "Barbarisation" of Roman provinces: not in the form of hairy Germans spontaneously popping with hairs full of butter and a breath full of garlic, but thousands of Barbarians were settled in Roman provinces either scattered or as communities. These settlers were coming from either defeated band or armies, or migrants/refugees coming from beyond the border and integrated as second-rate denizens under direct imperial control (at least legally) to make piece to the lack of workforce or military service. They might have kept their original cultural features (which, in Germania, were already importantly under Roman cultural influence) but overall were quickly submersed by the dominant Roman society, some of Barbarians obtaining really prestigious civilian and military charges, to name but Merobaud or Stilicho, but in the same time were more and more understood as being defined by their military service : this would be reinforced by the grant of a territorial autonomous status in the IVth, such as Salian Franks in Toxandria or Tervingi Goths in Moesia.
Not that Romans ceased to serve, but they didn't have the same "quality label", so to speak, especially troops raised trough fiscal service, even if quality troops, often found in comitateneses (although some allegedly lower troops could be transferred as pseudocomitatenses) on which Barbarian units were generally tied up : recruiting Barbarian and other quality units by giving them part of the supply and fiscal production of a territory might have been simpler than paying off a waged army.

The renewed Barbarian raids of the early Vth century quickly presented an existential threat to imperial authority and capacities in the West : the mismanagement of the Gothic crisis basically pointed that Romans weren't ensured of winning over them anymore, and had to deal with them no longer from a strong position. Alaric became a Roman general warlord, as much as a Gothic king and negotiated the service of his armies (joined by the lot of Roman destitute and slaves) on a personal level, the Gothic raids in the Balkans being as much a legacy of the IIIrd century Barbarians than what Roman armies tended to do when breaking away.

The limited resources of Ravenna were simply not enough in the late 400's, when the limes on the Rhine was broken : Stilicho had to choose between protecting Illyricum (arguably the traditional soft underbelly of Romania) and Gaul, choose the first and Barbarians were able to raid the westernmost provinces. This caused an anti-Barbarian reaction in both Constantinople and Ravenna, but Ravenna didn't have the means neither the politic skills to do so; and horribly mismanaged and underestimated the situation, the sack of Rome of 410 being essentially a slip-up into a difficult negociation, had nevertheless important consequences on the credibility of the state.

The further loss of whole provinces to Barbarian, in order to ensure having at least some quality troops against...well, other Barbarians; and critically the loss of Africa to Vandals, which was an essential part of the western Roman fiscal basket (Vandals having a different attitude than other Barbarians, and probably outright requisitioned lands of Proconsularis as theirs, instead of taking part of the production) meant that Ravenna had a really limited capacity to recover : an attempt was made to have Roman Gaul (on which Roman authority was fledging due to a perception of the Roman state taking taxes but unable to do its job) pay the annona, which wasn't met with enthusiasm, to say the least.
The dynastic disruption, the lack of clear legitimacy of emperors, and the choice of Italian senatorial elites to preserve and sanctuarize Italy even if it meant neglecting provinces : Northern Gaul gradually escaped imperial control since the 430's, and the death of Majorian led to the appearance of Roman warlords in Dalmatia, Northern Gaul and parts of Spain as quasi-breakaway states; where military rulers directly dealt with cities and possessores, sometimes acknowledging the western Emperor who had little authority over them.
Meanwhile, Barbarians inherited the state structures of the empire, building up their own palatial court and administration, with a more or less voluntary participation of provincial elites, that generally liked the idea of the emperor and Roman state; but not the reality of the mid-Vth century Roman state.

It's hard to point to a date and say "this is when the empire collapsed", but the traditional 476 is good as any : Late Roman Italy didn't went trough a political radical change, even if patricians and especially Odoacer underwent sort of austerity policy, the Roman state structures were still there as well a late imperial culture (which already began to evolve in Northern Gaul) and one that would be revivified by Theodoric the Great.

It's a really complex subject, and I fully expect people to rightfully point other possibilities, corrections or additions; but this was to give a general idea what happened.