Verify that the power cord is properly connected the outlet; the plug might have been removed to make way for another appliance. Do not connect the microwave to an outlet via an extension cord or plug adapter; the oven must be directly connected to an outlet.
Turn the outlet on, if possible, using the switch on the wall behind the counter or on the plug itself. Some microwaves have a separate power switch that must be in the correct position before you can begin cooking.
Unplug the oven, wait a few seconds and reconnect the power. A power surge could have temporarily rendered the oven inoperable. The oven should start up as normal when you connect the plug to the outlet.
See that the fuse or breaker protecting the oven's circuit is still active. Replace the fuse if it has blown, or reset the breaker to restore power to the oven. Stop using the microwave if it repeatedly blows fuses or trips breakers.