The sprocket bearing seized/broke, causing the gears to jam. Chain drive units put a lot of lateral force on that bearing, and when it goes, the gears all jam together.Our garage door opener suddenly failed. Lift Master brand. When you press the button (or remote) the light comes on but the head unit just hums. Almost like the gears are siezed.
I took the chain off the drive sprocket. No change in the situation. The door itself works fine when disconnected.
I used a channel locks on the sprocket to try to unstick the gears but there is no play there.
We have two standard size garage doors each with a separate opener. The units are likely original to the house (1989).
I had to replace the plastic gear sets in both shortly after we bought in 2000 after they both stripped within 6 months of each other, but that failure caused the motor to run but not move anything.
Now it seems the motor is trying to move the door but facing insurmountable load. (Again, note it is the same with the chain off the sprocket.)
It's also possible a motor bearing simply seized.