This happened to our 1998 Mystique once and only once, and it was the solenoid fuse. It is relatively easy to replace.
Now to get your car out of the Wal-Mart parking lot. If you look on the passengers side of the center console there is a slit near the shifter. Insert a long, metal object. (A thin flatblade screwdriver works well) Anyways, insert it until you feel it engage something and then push in a little. Next, pull the screwdriver towards the back of the car. While still pulling on the screwdriver, push the shifter button in and put the car in neutral. Then start the car and drive off. Note: the car MUST be in neutral to be started.
Your solenoid has either gone bad, or the fuse that controls your solenoid has broken. Your Haynes manual should tell you which is the right fuse to check. I don't remember which is the correct one, sorry. Anyway I'd check that first because it's easier/cheaper to fix than the solenoid. If the fuse isn't your problem then look at how to replace the solenoid in the Haynes manual, I have no info on this because I've never had to do it.
Hope you can get your car home, and hope you can get it fixed.
-James D.