Cycle-life spring and board exhaustion
That combination is now old enough that the spring has likely done its 10,000 cycles and the opener's circuit board is past the point where a homeowner should be poking at it with a screwdriver. A torsion spring holds enough stored force to snap a ladder in half if it lets go while you're working on it, and this isn't the kind of job where a video makes up for the right winding bars.
