It depends entirely on when you went back to. The joy of the medievalist is to be saddled with a period that stretches some thousand years.

So lets start early and work our way forward using three texts. Note all of them are literary and note actually representative of "spoken" language, but they will hopefully give you some sense of things. This is going to be largely non-technical and it is going to elide A LOT. There is a whole adders nest of issues including oral vs. written language, performative language, regional dialects, different socio-economic language practices, legal vs. everyday language etc. The development of modern English is a long and very complicated journey. Perhaps there are others here who can provide you with some more specific info. Certainly it might be worth your time to head on over to r/asklinguistics or r/linguistics and see what they have to add about such things as "the great vowel shift."

So here we go.

Anglo-Saxon, or Old English is a germanic language and would be largely unintelligible to you though it might sound familiar (in much the same way that say Dutch does, or less so German). I'll give you an example from that most famous of Old English texts, Beowulf, which dates anywhere from the 8th to the 11th century.

The Old English:

Oft Scyld Scefing sceaþena þreatum,
monegum mægþum, meodosetla ofteah,
egsode eorlas. Syððan ærest wearð
feasceaft funden, he þæs frofre gebad,
weox under wolcnum, weorðmyndum þah,
þæt him æghwylc þara ymbsittendra
ofer hronrade hyran scolde,
gomban gyldan. Þæt wæs god cyning!

And the translation:

Often, Scyld the Son of Sheaf took away the meadbenches, terrified the lords, with bands of raiders. After he was first found destitute, he took comfort for it, grew under the clouds, throve in honor, until each of those around him over the whale-road had to obey him, yield tribute to him. That was a good king!

Note that there are a few points that are largely intelligible: Þæt wæs god cyning! is one of the more famous lines and a favorite of teachers as it is so readily understood (Þ = th, æ = a in a of cat). "That was a good king!" But note also how much of the vocabulary is not familiar. Were you to be dumped back via time machine in England anytime from c. 500 to c. 1066 you'd have a hard time of it. It is, literally, a different language.

Onward! Scandinavian invasions in the late 9th and 10th century add their own twist to Old English, bringing new words and new flavor, but this would do little to improve your ability to understand, unless you happened to be Norwegian ;)

In 1066 the famous invasion by William of Normandy occurred and with it a whole new language entered into England, that of French (or really a variety of Old French tongues, also known as Oïl). From this period on we can see a new language emerging, commonly known as Anglo-Norman or Anglo-Norman French. Note that this was largely the language of the elite, the lower classes would have continued to speak Old English but even that language was influenced by French vocabulary and practice. Again, you would be unable to understand Anglo-Norman (even if you speak French) and equally difficult would be this period's English. These two traditions run on until around the 12th century.

Which brings us to Middle English! Middle English, typically dated from c.1100 to c.1450 (note again what a huge date range we are dealing with) is perhaps most famously associated with Chaucer, who wrote in the late 14th century. Lets turn to text again!

From the prologue of the Canterbury Tales:

Whan that aprill with his shoures soote
The droghte of march hath perced to the roote,
And bathed every veyne in swich licour
Of which vertu engendred is the flour;
Whan zephirus eek with his sweete breeth
Inspired hath in every holt and heeth
Tendre croppes, and the yonge sonne
Hath in the ram his halve cours yronne,
And smale foweles maken melodye,
That slepen al the nyght with open ye
(so priketh hem nature in hir corages);
Thanne longen folk to goon on pilgrimages,
And palmeres for to seken straunge strondes,
To ferne halwes, kowthe in sondry londes;
And specially from every shires ende
Of engelond to caunterbury they wende,
The hooly blisful martir for to seke,
That hem hath holpen whan that they were seeke.

And the "translation":

When April with his showers sweet with fruit
The drought of March has pierced unto the root
And bathed each vein with liquor that has power
To generate therein and sire the flower;
When Zephyr also has, with his sweet breath,
Quickened again, in every holt and heath,
The tender shoots and buds, and the young sun
Into the Ram one half his course has run,
And many little birds make melody
That sleep through all the night with open eye
(So Nature pricks them on to ramp and rage)-
Then do folk long to go on pilgrimage,
And palmers to go seeking out strange strands,
To distant shrines well known in sundry lands.
And specially from every shire's end
Of England they to Canterbury wend,
The holy blessed martyr there to seek
Who helped them when they lay so ill and weal.

Way easier to understand than Old English, yes? And yet note the non modern spelling and the unfamiliar vocabulary. And frankly, this is an easy part of the Canterbury Tales and the Canterbury Tales is popular in part because it is "easy" Middle English (though thats only one reason). Spoken aloud, this dialect of Middle English has "longer" vowels (in a non-technical sense) than you would be used to and sounds in some ways like a brogue, with a lilt to it. You could probably, with enough time, understand this, but it would certainly take some getting used to and you'd spend a lot of time asking Chaucer to repeat himself (much to his displeasure I'm sure). Go to Edinburgh and find some natives in a pub and see how much you can follow on the first go-round, that might be a comparable situation.

Towards the end of the Middle English period comes the so-called "Great Vowell Shift" which, as a non-expert, I won't go into but suffice it to say it is at this point that English begins to really sound like English.

In the 15th century Middle English transitions into what is now typically called "Early Modern English." This is the language of Shakespeare. I'm not going to bother quoting, because its basically unnecessary; I would hope that you've heard some Shakespeare! If not, get yourself some culture! Summer is coming, Shakespeare in the park is coming with it nearly everywhere, find some! Anyway, if you've encountered Shakespeare spoken aloud you know that it is simultaneously familiar and often difficult to follow at first. Part of this is that it is performative. An everyday conversation in Elizabethan England would not have been in verse and would not have been so "flowery." Spend 30 minutes listening the Shakespeare and your ear becomes attuned. You would have had little trouble in the 16th century understanding English. It certainly wouldn't be "familiar" (especially the vocabulary and diction) but it would be intelligible. The accents might still give you trouble, but compared to Old and Middle English, you would feel right at home.

So there is your grand tour from c. 500 to c. 1650. Hope you've enjoyed it.

This is one of the most frequently asked questions here. It's addressed in the FAQ.