My Speakers Dont Work :(

Hello,
i have a hp windows vista computer. earlier today my speakers were working fine. ive had no problems with my computer until it asked that i needed installments on it. i let it run its own thing & so then my computer restarted and all of a sudden a pop up appeared on my desktop and said itunes may not function properly. and that there are things missing. then the volume symbol that is on the bottom right corner of the screen said no audio output device is installed. when just a couple hours ago my speaker card was functioning properly. what do i do? im not that computer savy so i have no clue ...