The door stops partway and won't go further, or it closes unevenly with one corner dropping before the other. Sometimes there's a grinding or scraping sound as a roller fights the edge of the track instead of rolling inside it. On a double door, you might see daylight at one bottom corner where the panel has pulled away from the track entirely. If the opener runs but the door doesn't move the same way on both sides, that's a roller or bracket problem, not a motor problem.
This is different from a spring failure, which usually shows up as a door that won't lift at all, or drops fast and uncontrolled. An off-track door is a geometry problem - something isn't aligned with something else - and it's a different repair with different parts.