What happened after the 300 Spartans died?

Answer 1

At Battle of Thermopyles Sparta lost one of the Kings. Less than 8000 hoplites were left in city of Sparta itself. Year after 300 died, there was much bigger battle of Plateia,where full Spartan army fought the same enemy and won.

Soon after that, there was a devastating earthquake in Sparta that killed most of soldiers/citizens and destroyed the system itself.So after it Sparta never recovered as a full military society anyomore,and the military was too weak so slaves/helots started rebeling for the first time in their history.

Answer 2

At the pass of Thermopylae in 480 BC Leonidas & his bodyguard were superior in hand to hand fighting against the Persians. However arrows were thought to decide their fate....

Further:

Leonidas and his bodyguard were assisted by their 2,000 light infantry and 4,000 warriors from other cities, of course. When the naval battle which holding the pass was designed to precipitate, was lost, the reason to continue holding it evaporated, and Leonidas sent off the other city contingents to seel safety inside their city walls while he stayed to let them get away. The Spartan 2,300 selflessly went down to let them escape.