Diagnosis
The internal thermal overload protector or breaker has tripped, usually from a clogged coil causing the compressor to overheat.
How to Fix It
Unplug it from the wall. Wait 30 full minutes for the internal components to cool down and the protector to reset. While waiting, remove and check the side air filters for blockage. Plug it back in. It should power up.
If That Didn't Work
Check your home's circuit breaker for the room. This unit draws up to 7-8 amps on startup. If it's on a circuit with other appliances, it may have tripped the 15-amp house breaker.
Heads Up
If this happens repeatedly, it's a symptom of a failing compressor or a severe airflow restriction. Continuing to reset it will cause permanent damage. Time to call service.