Why did Sparta collapse?

Could you explain why the ancient Greek city-state of Sparta ultimately collapsed? Was it due to their rigid social structure, the decline of their military prowess, or the rise of other city-states like Athens? What specific factors contributed to this downfall, and how did they all intertwine to bring about Sparta's demise? Did the Spartans themselves have any inkling of their impending collapse, or did it happen suddenly and unexpectedly?