The Opener Will Not
Work through it in order, because four of the six causes cost nothing to fix and the expensive one is last.
Short answer
In order: power, the remote battery, the wall button, the safety sensors, the disconnect lever left pulled, and then the opener itself. A door that reverses is usually sensors or force settings; a door that does nothing at all is usually power or the logic board.
Work through it in order
Power: is the opener plugged in, is the breaker on, and is there a light on the unit. A tripped breaker or an unplugged unit is more common than anybody expects.
The remote: change the battery, and try the second remote. If the wall button works and the remotes do not, it is the remotes or the receiver.
The wall button and its wiring, which is a low-voltage pair that can be damaged or disconnected.
The safety sensors: are both indicator lights on and steady. A blinking sensor light or a blinking opener light is a misalignment or an obstruction.
The manual disconnect: has the release cord been pulled, leaving the trolley disengaged from the chain or belt.
And the opener itself: the logic board, the capacitor, the motor or the gear and sprocket.
The blinking light code
Most openers signal faults by blinking the courtesy light or an indicator a set number of times. The manual decodes it, and the manual is usually available by the model number on the unit. Counting the blinks before calling anybody saves a visit surprisingly often.
If it reverses instead of closing
The safety sensors are the first suspect: misaligned, dirty, or with something in the beam.
A sensor knocked by a snow shovel, a bike or a bin, which is the most common version.
Sun shining directly into a sensor, which can defeat it.
The down force setting too low for the resistance the door now presents, which in cold weather is the usual story.
An obstruction at the threshold — ice, a bin, a cable — which is the sensors doing their job.
And a binding door, which increases the resistance and triggers the reversal even with correct settings.
Repair or replace the opener
Repair where the fault is a specific part: a sensor, a wall button, a remote, a gear and sprocket, a capacitor.
Replace where the logic board has failed on an older unit, because the board cost approaches the cost of a new opener.
Replace where the opener is old enough to lack rolling code security, which is a genuine reason on its own.
Replace where it lacks photo-eye sensors entirely, which on a pre-1993 unit it may do, and which is a safety matter.
Consider replacing where the door is being reworked anyway, because the labour overlaps.
And in either case, have the door balance checked, because an opener working against an unbalanced door fails early whatever you fit.
Common questions
The door reverses when closing. Why?
Usually the safety sensors, or the down force setting being too low.
What should I check first?
Power, the remote battery, the wall button, the sensors, and whether the release cord has been pulled.
When is a new opener worth it?
When the logic board has failed, or where the unit has no rolling code or no photo-eye sensors.