Hi,
Check that the door interlock switches [https://www.searspartsdirect.com/model/1uvkkqf38s-000432/ge-peb9159dj1ww-countertop-microwave-parts|part #20 & 21 -oven cavity parts diagram] are operating correctly and that the switch levers (part #20) and also the interlock switch bracket (part #24) are OK and not broken or loose
The interlocks are a safety device that prevents the microwave from starting if the door is not closed and also stops the microwave if the door is opened while it is operating.
'''Be safety aware''' ''when working inside a microwave oven as the HV capacitor can store >6000V DC for months even if the power has been disconnected for this length of time. It needs to be discharged correctly before commencing any work inside the oven after the cover has been removed. This'' '''voltage can seriously injure you or worse.''' ''If you don’t know what you’re doing then don’t do it!''