If you have an independent sound card, it's likely that whomever installed your gfx card unseated your sound card from the PCI slot. You simply need to open up your computer and make sure that all of the cards (graphics, sound, etc) are all seated properly and have all their cables connected.
Note: When you open the side of your computer, the cards that are seated in slots on the largest board are the PCI cards. The one with the largest dies and fans is likely the graphics card, the sound card is the other card. If there is only one card then your sound is built into your motherboard. In that case you may have a hardware failure.