Make sure the power cord is connected. Then wait a moment for the power button to light up before pressing the power button on the projector or the On button on the remote control.
If you've set the Control Panel Lock, the control panel buttons are disabled; you must use the remote control to turn on the projector. See Locking the Projector.
If you've set the Child Lock, you have to press the power button for 3 seconds to turn on the projector, or use the remote control. See Locking the Projector.