Unplug the motor unit. Use the screwdriver to remove the four screws that hold the motor cover to the chassis of the garage-door opener.
Shine a flashlight on both the motor drive and controller boards. Look for burn marks or melting -- signs of electrical damage.
Circuit boards that show signs of electrical damage will need to be replaced.
Put the motor cover back on. Pull the red handle that extends from the black, plastic carriage that travels along the rail.
Manually lift the door to the halfway point. Push up on the metal handle the red cord is attached to.
This will lock the carriage back into place on the screwdrive rail.
Plug the motor unit back in and activate the garage-door opener. Operate the door, if it will move, through at least three up-and-down cycles.
Compare the movements of the garage door with the components of each board. If the unit refuses to activate at all, has no power, or if the light bulbs display erratic symptoms, then the motor drive board will need to be replaced.
If the unit is active but will only travel short distances, or if it ever runs at the accelerated speed on the downward movement, then the controller board needs to be replaced.