A microwave with a door that won't close properly won't run — by design. The safety interlock system requires the door to be fully latched and all three door switches to be activated before the magnetron can receive power. A door that appears closed but hasn't engaged all switches will prevent the microwave from starting.
The door latch system consists of the latch hooks on the door, the catches (holes in the microwave body the hooks engage), and three interlock switches behind the door frame. Common failures include broken latch hooks (plastic tabs that snap when the door is closed hard), worn interlock switches (the switches actuated by the hooks), and broken or bent door handles.
Door handle replacement and interlock switch replacement are the most common microwave door repairs. Both are straightforward fixes that restore normal operation without needing to replace the entire unit.