Emergency release cord stuck?

The red cord is attached to a lever on the trolley, the part that rides on the rail and connects to the door arm. When you pull it, the lever lets the carriage slide free. If the opener is pushing hard on the carriage with the door closed, or the door is pulling it with weight, that pressure can lock the lever so the cord will not release. A little tension on the cord and a stuck lever feel the same from the floor.

Other reasons are simpler. The cord can tangle on the arm, a knot can jam in the handle slot, or the cord can fray and snap. Dust and old grease sometimes gum the lever, and a bent release handle will not open even when pulled straight. On some units the lever is plastic and gets brittle with age.

The bigger warning is when the cord pulls but the door still feels very heavy. The opener may not be what is holding it. A broken spring or bad cable means the whole weight of the door sits in your hands once the opener is released, and it can fall. So a heavy door and a stuck cord together call for a stop and a call, not a harder pull. Look at the lifting cable and springs from a distance for any visible gap or sag, but do not touch them.

A stuck cord is a small part problem on most openers. If the trolley is cracked on an older unit, the parts are still often fine. If the spring or cable is the cause, repair that part first. See repair or replace guide if the opener is also old.