Before You Call For Service

Confirm that there is a problem, particularly if you hear about it second hand.
Check the STOP switch or button. Be sure it's in the normal "run" position.
Check all keyed or restricted switches to be sure they're in their normal "run" positions.
Check for doorway obstructions. This can include foreign matter in door sill grooves or objects blocking complete closure of the doors.
Be sure you're receiving full power from your utility. Have a qualified person check to see if there is a tripped breaker on the main power supply.