Although this may sound stupid check the speaker cable is still plugged in to your sound card.
I was called out a couple of days ago to someone who said his sound wasn't working. I set his computer up.
What I found was, he had dropped a pen down the back of his desk and pulled the computer out to get it back. This in effect pulled the speaker lead out of his sound card. I have a call out fee but because it only took me a couple of minutes I didn't charge anything.