One side is not working.

Try swapping the L and R side speaker wires. If the sound switches sides, then you know the headunit/amp is the problem. If the sound stays on the same side, then you know the speakers or speaker wiring is the problem. If you can, run a new speaker wire from the headunit/amp to one of the speakers that's not working. That will help you troubleshoot if there's a wiring problem or a speaker problem. And of course double-check your balance and fader settings first!