You are correct. The metal shield acts as a part of a Faraday cage (in conjunction with the rest of the body of the oven) to prevent any microwave radiation escaping.
You could Google " how to access microwave oven door locks". The answers might show you how to remove the trim on the inside of the door (it varies depending on brand) which once removed will hopefully allow you to see why the shield has moved. With a bit of luck the reason might be obvious and simple to fix.
Please ensure that you have unplugged the oven from the power before you attempt anything. When replacing trim etc ensure that you do not snag or pinch any wiring.
Also follow the old adage " if in doubt, don't"