Start with the photo eyes, because it is usually the photo eyes
Two small sensors face each other about six inches off the floor on either side of the opening. If the beam between them is broken or misaligned, the opener will refuse to close and will normally flash its lights or beep at you. Check for a bin, a bike or a coiled hose sitting in the beam. Then wipe both lenses. In Las Vegas this matters more than most places, because fine dust settles on the lens and reads as an obstruction. Both indicator lights should be on and steady. A blinking light means misalignment: loosen the wing nut, move the sensor gently until the light goes solid, and retighten.
The door closes then immediately reverses
If the door reaches the floor and bounces back up, the close travel limit or the force setting is out. The opener thinks it has hit an obstruction because it is being told to keep pushing after the door is already down. The adjustment screws are on the opener housing and are usually marked. Move them in small increments. If you find yourself increasing force substantially to get the door to stay down, stop: you are compensating for a mechanical problem and disabling a safety feature to do it.
The door closes only when you hold the wall button
This is the opener telling you it has decided closing is unsafe. Holding the wall button is the deliberate override for exactly that condition. It confirms the fault is in the safety circuit, which means the photo eyes, their wiring, or a sensor damaged at the bracket. Do not leave the door operating this way, particularly if there are children or pets in the house.
Nothing happens at all
Check the obvious in order: is the opener still plugged in, has the breaker tripped, has the wall lock or vacation mode been switched on, and does the wall button work when the remote does not. A remote that has stopped working while the wall button still works is a remote problem, not a door problem, and it usually starts with the battery.
The door is crooked, binding, or grinding
Stop here and do not keep cycling it. A door sitting unevenly, dragging in the track or making a grinding noise has a mechanical failure: a cable off its drum, a broken spring, a bent track or a seized roller. Running the opener against a mechanical fault is how a bent panel becomes a replaced door. Leave it closed and call someone.
When to stop and call
Call rather than continue if you can see a gap in the spring, a hanging or frayed cable, a door sitting visibly out of square, or if the door will only close on the held wall button. Those four are the ones where carrying on causes damage or gets someone hurt.