Our is a Kitchen Aid but I am sure they are all pretty much the same.
Start your investigation by removing the bottom plate. There should be a belt or chain going around 3 gears. Mine has 2 metal and one plastic. The 2 metal ones are on the ends of the rods that the ram runs up and down on. The plastic one is on top of a big speed reduction gear that is turned by the motor gear. Don't pinch your fingers but see if you can turn the big gear by hand. All gears should turn. If not check each gear for broken teeth, or bad bearing & races under the 2 rod gears. This area gets a little expensive. The big gear around $35, the 6 washers and 2 bearings $100, I can't remember how much the metal gears are. If this all seem ok go to paragraph 2.
Been trying to find a repair manual for mine but no luck so far. Ok, for your problem and this is strictly a guess. If the gears at the bottom are turning and also turning the long rods then your problem is probably the two items called the Power Nuts. With the top off look down the sides where the Screw Power Rods are located. You will see a plastic sleeve on each rod. These are threaded and are bolted to the ram. As the rods turn these ride up & down the rods lowering or raising the ram. There could only be 2 things wrong with these Power Nuts, 1. They are stripped. 2. They have broken away from the ram.
I would think to fix them you will have to unbolt the rods from the botton and pull the entire unit up through the top. Then you could replace the broken part and be able to open the door.
If I find a repair manual I'll let everyone know. Good luck, Mike.