Prosperous :

Adverse : preventing success or development, harmful, unfavorable.
Advanced : far on or ahead in development or progress.
Retarded : less advanced in mental, physical, or social development than is usual for one's age.
Affluent : having a great deal of money, wealthy.
Impecunious : having little or no money.
Prosperous : successful in material terms, flourishing financially, wealthy.

So, antonym of Prosperous is Impecunious.