Sound not working

Hello,
Hi there! right basicallly my mum moved my computer around so i'm guessing she unplugged a few things etc, altho everything does seem to be in it's correct place, also the speakers do light up when you switch them on, however sound doesn't seem to come out, only for itunes for my music :S other things such as videos it doesnt work!!! it's just really frustrating now cos i just havn't got a clue how to fix this! please help!