I had this problem. It turned out to be a loose connection in the handle. This was after I had changed the mechanism and solenoid, which did not cure the vault. To get the rear door card off with the door closed is a mission. First you must undo four screws that hold the card on the bottom. All four are barstewards to get to and a flexible bit holder would be the best tool or a right angle screw wotsit (like Allen key). The first time I opened the door I physically managed to wiggle a right angled bit of coat hanger to actuate the solenoid mechanisms, but the best way to open the door is to indentify to wired to the solenoid and run 12v to it and the door will pop open.

now you can check the connections to the handle and the door mechanism. make sure sure all works before putting the door card back on and shutting door again. I put a button to the door opener, so that I can open the door from the inside should this happen again.

Go to you tube for video to give better idea.

can take some pictures if needed further.

G