If the guys who worked on your a/c had to work under the dash (which is necessary for some a/c components), they could have loosened a connection or damaged the speaker wire.
To check, try the following methods:
1) Remove the radio (search for instructions online), pull it out of the dash far enough to release the antenna and wiring connectors from the back (usually by pulling them straight out), then remove the stereo entirely. Then you can check the connectors and the wiring in the console for any obvious problems.
2) You didn't say which speaker quit working, but if the a/c guys had anything to do with it, it was probably one of the fronts. Check the wiring by removing any cover panels under the appropriate side of the dashboard, then sticking your head under the dash with a flashlight and looking for anything wrong with the wiring. IMPORTANT NOTE:
Any time you're working under the dash, disconnect the car battery first to prevent accidental activation of the air bags.
3) Depending upon the age of the stereo, it is possible (though fairly unlikely) that one channel has gone dead. To test this while the stereo is out, you'll need to:
--provide power by running a wire from the positive terminal of the car battery to the appropriate terminal on the back of the stereo, which usually corresponds to a red wire on the connector.
--provide a ground in the same way, from either the battery's negative terminal or a known good ground point. The ground wire from the stereo is usually black (sometimes dark brown).
--provide a substitute pair of wires to the dead speaker (or any other car speaker you've got hanging around). Usually, each pair of speaker wires from the stereo is a wire of one color and another wire of the same color with a stripe (for example, purple and purple/white). Figure out those connections from the stereo, then connect the other ends of the wires to the speaker.
--turn the car key to "accessory," then turn on the stereo and see if the test speaker works. If it does, you know the problem is the wiring or connectors under the dash. If it doesn't, then the problem is the stereo itself.
--if the stereo doesn't turn on at all, it probably means that you've connected power to the "keep alive" terminal that keeps the radio's memory alive. Look instead for another brightly colored wire (often with an inline fuse, usually NOT blue), and try providing power to that terminal. Sometimes, a sticker on the stereo will show the proper electrical connections, or you could try looking online.
4) Or, you could call up the guys who worked on your a/c. First, ask them exactly what they repaired/replaced on your a/c, then ask if any of their work was under the dash. If they say yes, then tell them you think they screwed up your stereo wiring. If they admit that's possible, ask them to fix it themselves, or agree to pay the cost of a car audio installer to fix the problem, if the installer finds out it's probably their fault.
The following diagram shows a typical auto a/c system. The components usually found under the dash are shown at lower left. If the a/c guys repaired or replaced the expansion valve or anything related to the evaporator, that means they were almost certainly under the dash. (On GM vehicles, however, the expansion valve is often in the engine compartment, but the other components are similar to those shown here.)