I made a cake and covered it in fondant. the next day I went to dust the fondant and could'nt cause it was sweating to much. WHY

Was it very humid where you live?

Was it raining?

Had the cake been in the refrigerator or freezer, or some other very cool room ?

"Sweating" is just condensation. It usually happens when a cold cake hits warm, moist air. If left alone, the cake will warm to the room temp, the condensation will dry on it's own (don't touch the cake during this time) or a fan can help--UNLESS, it's rainy or unusually humid and then, all bets are off.

HTH
Rae