Gateway- built-in laptop Speakers not working

One minute my speakers are working fine and the next minute they are not. i have a gateway laptop. windows vista basic. My mute button is off (not checked) my volume is on 100, i have restarted my computer several times.....still nothing. but sound does come out if i use external speakers or headphones.... what could be the problem? please help me. My Gateway's right speaker stopped working a few days ago after listening to some music. When I use my ear piece in, I hear sounds from both speakers, but when I disconnect the ear piece I still only hear the left speaker.