Make sure the power cord is connected. Then wait a moment for the power button to light up before pressing the power button.
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.