You may want to hold down the power window switch in the up position (use the switch in the centre console), while having another person open and slam close the affected rear door. This may help loosen the stuck window motor (that's what happened in my old 300E) and the window may go back up.
Give it a try and see if it works. If not, you will probably need to remove the door panel to test the motor to see if it moves when you apply 12V to the motor. If it doesn't move, you may need to replace the motor.
Hope this helps.