The tl;dr is that Sparta didn't keep pace with innovations to warfare at the same time that they had a serious population crisis, leading to the decline of Sparta and the rise of Thebes. It would eventually be absorbed by Rome.

The Battle of Leuctra, in 371 BC, was the first defeat Sparta had in regular hoplite warfare since a one-off in 390. A Boeotian army led by Thebes fought Sparta and it's allies, and Thebes used unconventional tactics, resulting in a crushing defeat for Sparta.

By the time that happened Sparta had been really weakened. It's greatest weakness according to Aristotle and most historians was a lack of manpower. By 371 there were no more than 1000 adult male Spartan citizens all told. An earthquake in 464, losses in battle, and Spartan inheritance law, all led to this small pool of manpower for hoplites. In the Battle of Leuctra, Sparta fielded 700 men, of which 400 died including one of it's kings.

In late 370, Epaminondas, the victorious Theban general, invaded the Peloponnese with 30-40,000 men. Many of Sparta's erstwhile allies had already defected, but they wouldn't have been able to stop him anyways. He got so close to wall-less Sparta that the fires of burning crops and buildings could be seen by the Spartans. He didn't take Sparta, but probably destroyed their port. At this time, the Messenian helots (a slave class that did all of the labour, freeing Spartan citizens to train for war) revolted for the last time. The goal of Epaminondas was to make sure that this revolt wasn't put down, and he oversaw the reconstruction of New Messene. Now the Spartans had a tiny army, and no slave labour. He also supervised the construction of Megalopolis (The Great City), by combining forty smaller communities in Arcadia, and made it the capital of that region. It's placement directly threatened any northward passage of a Spartan army, impeding any future interventions by Sparta.

The Peloponnesian League (the Spartan hegemony) was dissolved as an effective instrument of Spartan power within a few years, and Sparta was forced to seek an alliance with Syracuse for protection.

By the time King Phillip II of Macedon was subduing the Greek city states, he didn't bother with the trouble of conquering Sparta. Apparently he felt that excluding Sparta from his Corinthian League would ensure the loyalty of all the anti-Spartan cities. In the post-Alexander Hellenistic world, Sparta struggled for relevance, but staved off conquest. Reforms were attempted to give slaves land (and therefore citizenship and the ability to be a soldier), but they were undone when the reforming kings were killed or forced into exile. In 192, Sparta was forcibly entered into the Achaean League, giving it a master and ending it's long held autonomy. It's first walls, just newly built were torn down and the last of the famous Lycurgan laws abolished (although by this point they bore little resemblance to the past). The Achaean League in turn was subject to Rome, meaning Sparta was now indirectly a Roman subject.

When Roman general L. Aemilius Paullus was conquering Macedonian Greece, he stopped at Sparta to pay respect to it's ancestral way of life, though there was little of this left beyond a peculiar style of clothing and a hair style the men wore. The city would later revolt from it's Achaean masters, and be beholden to Rome directly.

Under Roman rule Sparta turned it's city into a sort of theme park, like a museum of it's glorious past. It produced it's first historians, to record it's history for posterity. A hostel was built for visiting Roman tourists, and laws were passed to replicate the harsh past of Sparta, just so tourists could see it.

Beyond this point, someone else will have to take over, my resource ends here. I took all this from the closing couple of chapters of Paul Cartledge's book "The Spartans". If anyone takes issues with any details, let me know.

edit: I just realized I missed the point of OP's question, and only really partially answered it at the end. Egg on my face.