After you pull the red cord, the opener only lifts the door again once the trolley locks back onto the carriage. Close the door by hand, then reconnect. You pulled the handle because the power was out or the opener was dead. Now the motor runs and nothing hooks.
That cord is the emergency release. It pops the trolley free from the rail carriage so you can move the door without the opener. Getting it to catch again is the part people skip. The opener will happily run along the rail all day while the door stays wherever you left it. That is the same symptom people describe when the opener hums but the door stays shut. The motor is not broken. The door is not broken. They are just not talking to each other.
If the door still rolls by hand
Close the door by hand first if you can. Stand clear of the opening. Look up at the rail. You want the trolley, the little shuttle that rides the rail, to meet the carriage connected to the door arm. Some units click back when you pull the cord toward the opener while you nudge the door. Some need you to lift a lever on the trolley until it snaps. Brand stickers on the motor head usually show a tiny diagram. Follow that picture, not a random video for a different opener.
If the cord hangs limp and the trolley never meets the carriage, the door may be open or closed at the wrong spot for that latch. Roll the door a few inches by hand so the two parts line up, then try the reconnect move again. Keep your head out from under the panel. Keep kids and pets clear of the opening while you do this.
If the door feels like a brick
Stop. That is spring territory. A healthy door should lift with steady effort, not like dead steel. A bang earlier in the week plus a door that will not budge means the spring may already be gone. Do not force it. Leave the door down if it will stay, use the house door, and get a shop. Trying to reconnect a trolley on a door that will not move by hand just burns the motor the next time someone hits the remote. See the broken torsion spring note for that fork.
If one side lists or the door binds
A roller hanging out of the track or a cable that frayed or jumped a drum can leave the door crooked after you freed it by hand. The trolley may click back on, then the opener fights a bind and stalls. Or the door twists when you try to roll it to the latch point. Stop. Do not kick the bottom panel. Do not shove a roller back in with a screwdriver while the opener still has power. Leave the door where it sits if it feels safer that way, and walk through the house.
What it is not
This is different from a remote that does nothing while the opener stays quiet. Silent remotes are batteries, lock mode, or a dead board. Here the opener still runs. You can hear it. The missing piece is the latch between motor and door.
It is also different from an opener that grinds like gravel. Grinding usually means a stripped gear inside the opener while the door still travels a little. A freewheeling rail after a cord pull is just a disconnect. Quiet motor head. Door ignored. No rock blender sound from the housing.
What not to do
Do not keep cycling the opener hoping heat will make the latch catch. Each wasted run cooks a motor that may still be fine once the trolley locks. Do not climb a ladder under a hanging door to poke the carriage with tools. Do not cut the red cord or tie it short so it cannot hang. Do not force a brick heavy door or a listing door just to make the click happen.
I am not a contractor. This is what shows up when someone yanked the red handle to free a car and never locked the trolley back on. A shop that works on garage doors will reconnect it in a few minutes and check the spring balance, the tracks, and the cables before they hand the opener back to you.
Manufacturer notes on emergency release use live on DASMA's garage door opener page. Those tips cover disconnects and reverse rules, not a full door rebuild.
The U.S. CPSC garage door operator page covers federal rules for automatic openers.
If the cord still will not catch and you want someone local to look, get a quote.