Computer sound help!!!?

my sound on my computer isn't working. it has done this 3 times before. the first time i tried the volume thing and it didn't work. then i went to get my brothers help and he turned the nob too and it DID work! then the next time the volume didn't work, i made sure the nob wasn't working. then i got my brother and even showed him it didn't work, BUT IT DID! grrrrr---and now it's not working and i really need the volume but if I ask my brother, and it starts working, he'll be mad!!! any help?