Check the garage door to ensure no locks or bolts are preventing the door from opening.
Make sure any wall switch that supplies power to the opener is in the "On" position.
Allow the unit several minutes to cool if it has been used numerous times in succession. If the unit overheats, it will trip an overload switch that resets itself.
Press the reset button on the back of the opener chassis.
Inspect the lock switch. Move the switch to the "Off" position, away from the label that says "Lock."
Make sure there is nothing obstructing the path of the door if it will not completely close.
Replace the battery in the transmitter if it is failing to control the opener. Slide the battery cover off to access the battery.