Computer upgraded, now speakers wont make sound?

I recently got my computer upgraded with a new hard drive. The only problem now is, my speakers or headphones won't make any sound, they are not broken and they are plugged in right. I checked to see if they were on mute, but it just sais no audio divice found. I remember this happening years ago on my old computer, but have no idea how i fixed it, i've been trying to figure it out for days.
If anyone could tell me with in the next couple of days I would be extremely grateful as I am going to realy need it in the future.

Scott
September 2006