A microwave that is completely dead — no display, no response to any button, no light when the door opens — has lost power to the control circuit. Before calling for service, verify the outlet is working (plug in a different appliance) and check whether the circuit breaker has tripped.
If the outlet is confirmed working, the most common internal cause is a blown line fuse — a small ceramic fuse inside the microwave that blows after a power surge, door slam, or internal component failure. The fuse is inexpensive and replacement is straightforward, but the root cause of the fuse blowing should be identified to prevent a repeat failure.
Door interlock switch failures can also cut all power — the microwave's safety circuit requires all three door switches to be functioning. A thermal cutout (an overtemperature safety device) can also open and cut power if the microwave has been running excessively or ventilation is blocked.