WHY DOSENT MY ACER NOTEBOOK HAVE ANYSOUND?

So i have this acer notebook & the noices play but not the music fom youtube or the sound you hear when u have a facebook IM also yesterday night when i tryed to use youtube it said i didnt have the adobe flas that i needed a newer version in order to play the videos ... but a cupple of hours befor that everything was fine youtube was playing & i was on facebookbeing able to hear the IM noises .... what do i do ??? HELP!!!