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Microwave Ovens (and combo hoods) can lose screen from a voltage outage or if you experience a brown out.
First, try and disconnect the microwave from the wall outlet in the cabinet above it. Then, after a minute reconnect it. If this resolves your problem you're all set for now. It will likely happen again. So, maybe a good quality surge protector could help.
IF it does not resolve your issue, you will have to take it apart to test a couple of things before deciding a replacement part. Have you done something like this before? Do you have a basic volt meter?
I hope this helps. Let me know if you have any other questions.