Are we talking about onboard sound here, or a seperate sound card? If it's a sound card, check that it's seated properly, and that the pc's recognising that it's plugged in.
If you'd unplugged the speaker leads, double check that they've been put back in the correct slots. For 2 speakers, that usually means the green slot, for 5.1 surround that usually means green=front; orange=center/sub; black=rear.
If you could let us know what you've already tried, it would help speed up the process.