The most common cause is an improper power connection. Check that the power plug is completely inserted into a 120V AC wall socket. Don't use a broken plug or a loose power socket.
If the plug is secure and the socket works, unplug the unit. Wait 10 seconds, then plug it back in. You should hear a beep and see all the control panel indicators light up for 3 seconds. That's the standby mode. Press the ON/OFF button on the control panel or the remote to start it.
If that didn't work, inspect the power cord for damage. Don't use it if the cord is cut, twisted, or has been dragged excessively. To repair the power cord or plug, you must contact an authorized repair center.
Heads up: Always disconnect the unit from power before any cleaning or maintenance to eliminate the risk of electric shock.