It is defiantly possible to only have one channel go out on a stereo, and the tweeter and midrange are on the same channel in our GP's. A could of things I would check though, like everyone else said, make sure your getting power to your speakers, easiest way is to grab a speaker and test it, or just use a millimeter. If you rant getting power, then follow the wires back to the stereo and make sure that harness is secured, maybe even re-seat it. If you are, which is a good thing, check the connections between each speaker and the wires, if its the speakers, buy better ones!
Best of luck!