Sounds like the torsion springs have failed on the door. I wrote about the repair in my post Replacing Microwave Torsion Springs and then while you are at it, you may want to go ahead and replace the turntable motor. It is hard to explain how to test that motor as it is not easy to get to, but while the unit is out to replace the springs, pulling it out a little more to do a resistance check of the motor shouldn't be much more work. If the motor reads as an open circuit, then it is bad, but I wonder if the motor is bad, or the coupler to the motor is stripped. You can lift up the plate and pull up on the coupler. See if it looks stripped or the plastic is damaged. It is also possible there is a control or wire harness problem but that requires some more trouble shooting and is rather involved. Start with the easy stuff, then go from there if need be.