Multimeter - check to see if you are getting voltage at the lead that connects to the window motor. If you are, perhaps a bad motor, if you aren't perhaps a wiring problem.
You should be getting +12V at either wire that connects to the motor, violet with a white strip or black with a white strip, depending on whether you are trying to put the window up or down. Dunno which is which, but it's pretty easy to figure out.
If your other windows are working, I would suspect the motor.