A garage door that will not close is usually the opener refusing to send it down rather than the door failing mechanically. Watch the first two seconds after you press the button. Nothing happens, a light blinks on the motor head, the door drops an inch and stops, or it runs all the way to the slab and rides straight back up. Those four behaviors sort the fault before anyone picks up a tool.
Blinking light and no movement: the safety sensors
Two lenses sit about six inches above the slab, one each side of the opening, and they have to see each other for the whole close cycle. Break that beam and the opener will not start down. Most units show a steady LED when the pair is aligned and a blinking or dead LED when it is not. The causes are usually dull ones: a recycling bin leaning on the bracket, a web across the lens, a bracket knocked out of line by a bumper, or a nicked low-voltage wire where a staple went in during a remodel.
Low winter sun is the one people miss. A west-facing opening at half past four in January can put direct light into the receiver and swamp the beam. If the door closes at nine in the morning and refuses at five in the afternoon, that is not a failing board. That is the sun.
Full travel then a reverse: seal, slab, or down limit
An opener that reaches the floor and then reverses thinks it hit something. Start with the bottom seal. Richardson runs about 20 days a year at or above 100°F and about 29 nights at or below freezing, which is roughly a 130°F swing across a year on a south or west facing steel door. Cheap vinyl goes hard and stops compressing, so the door lands on a rubber bar that no longer squashes, motor load spikes, and the safety reverse fires. EPDM handles that swing better, and replacement weather seals at the bottom and the stops run an estimated $105 to $235.
Next the slab. Blackland Prairie clay under this city shrinks and swells with moisture, and a slab corner that has lifted a quarter of an inch will stop a door dead on that side. Last, the down limit. The opener stores a floor position in memory, and if that position sits below where the door now lands, the motor keeps pulling into the concrete and reverses itself. Resetting it takes minutes once someone has confirmed the door itself is free.
An inch of travel then a stop
That one is physical. Something is on the track, a roller has come apart, or the door is fighting its counterbalance. Sight both tracks with a torch. Dried mud on the track lip, a bike pedal or a bag of potting soil accounts for a fair share of it. If the tracks are clear, pull the emergency release with the door down and lift by hand. A balanced door takes about ten pounds of effort and holds itself near chest height. One that slams or creeps has a counterbalance problem, and no opener setting covers that for long.
What you can touch and what you cannot
Clean both sensor lenses with a dry cloth, sweep the tracks, check the bracket bolts are still tight, clear the seal channel. All fine. Torsion springs, lift cables, cable drums and the bottom brackets are not. A wound spring holds hundreds of foot-pounds of energy and the bottom bracket is bolted directly into that load. Those parts are professional work, and the reason is not insurance wording. It is that the energy comes out faster than a hand moves. The Consumer Product Safety Commission also asks homeowners to test the reversing feature monthly, which is worth doing the day a door starts behaving oddly rather than waiting for the calendar.
When it is the opening, not the door
If the door seals on one bottom corner and leaves daylight at the other, look past the door. The City of Richardson publishes its own homeowner foundation guidance and requires a permit for foundation repairs of four or more piers, which tells you how ordinary slab movement is here. Water uniformly on all sides, keep downspouts at least three feet out, and remember that trees or large shrubs within 20 feet pull moisture unevenly. A racked opening needs the tracks and the header looked at together, not a fresh bottom seal.
A door that will not close and will not stay closed leaves the garage open to the street, and that is one of the few situations where the after-hours rate earns its money. If the door is down and secure and the only fault is a reverse on the last inch, book the morning instead. Same repair, standard rate.