my speakers for my computer dont work.. i know the speakers wrk find if i

hook them up to a mp3? ok my friend came over and i let him use my computer i left for 3min and hes like"dylan i cant listen to the music somthings wrong with ur computer"i waz like wtf u do so i hooked his mp3 up to it and the music played but when it comes to my computer it dont work how can i fix this??
ok so basicly my speakers work but my computer wont play anything throught them i look at the mute and the volume stuff and everything is working exsept its not playing and i tryed system restore and it still dont work.. i need help bad music is like my life