Take the regulator out of the door (with the motor attached, it can be removed as an assembly) and see if the motor will turn and activate the mechanism if not in the door. The regulator may be shut, rusted or the connections to the motor may just be corroded, or the motor can be shut too.
The door is easy to take apart, You remove the handle by removing the Screw that holds it to the opening mechanism, then you have a single screw holding the whole door panel. Grab the panel, slide it upwards until it unhooks from the door. Peel away the internal plastic lining, but take care not to tear it as you want to be able to reuse it. Then you have to loosen the screws holding the Glass into the regulator, and there are 4 bolts holding the regulator assembly in the door. You can wait to unplug the motor until after you have removed the bolts as the connector is easier to reach. Pull the assembly out of the door and inspect for breaks or if the "X" will manually slide in its guides. The you can check for the motor to see if it activates when powered and if the power is actually getting fed all the way to the motor.
If the motor is shot, I suggest replacing the whole assembly as the regulator part is not the costly piece and it usually is what got the motor to fail in the first place as the regulator mechanism as a tendency to Jam after a long while.
Hope this helps