I think the 'buzzing' noise I've clumsily described is actually the sound of the door-opener's motor running constantly.
The door went smoothly up and down as usual when I parked the car in the garage on Friday. Afterwards, all was well and the motor was quiet. On Saturday morning, I walked into the garage through the back door and it sounded like the door-opener's motor was running. This had occurred suddenly and mysteriously overnight. The garage door wouldn't open and I had to disconnect the power to stop the motor noise.
I've been opening the garage door (a tilt type) manually for the last few days by pulling the release cord. That works fine. I don't leave the opener plugged in because of the constant motor noise but each morning, I re-connect the power to see if the opener has fixed itself. No such luck.
A stuck control relay sounds plausible but so far it hasn't rebooted. Is there something I could try to 'unstick' it? Is it possible for a novice to fix or replace the relay?