I agree it sounds like something got knocked loose, but if you don't know whether you had a sound card, you probably didn't. If you're plugging your speakers into the same place you were before, then the card that was removed was definitely not a sound card. Most likely you have one of the RealTek on-board systems. If that's the case, the repair shop may have done physical damage to your motherboard. If you're able to open your case, at least have a look and see if anything is obviously disconnected (if you haven't done this before, have a look in your computer's manual).

Can you tell us more about your system? If it's off-the-shelf, then make and model. If it's custom, the motherboard make and model.

In any case, I sympathize. I was having similar problems just a week or so ago. My computer was freezing up frequently, I think because it was in too cold a corner and mostly idling, so not producing a lot of heat. I moved it to a warmer area, and the freezes stopped, but I lost all sound :S I went through all the uninstalling and reinstalling of drivers only to discover that I had a bad line-out wire, one of three connecting the speaker system to my sound card. These things can be a pain in the butt, but there's almost always a solution.