Sears Craftman garage door opener won't close anymore

I'm having a problem with my Sears Craftsman garage door opener. For some reason, the garage door won't close anymore. (Now it's only a garage door _opener_). Neither the switch in the garage nor the remote keypad will close it.
The light on the opener works. If I put something in front of the safety sensor, the opener light blinks, as expected, so I do't think it is a problem with the safety transmitter or sensor.
There is a yellow LED on the opener that blinks 5 times, pauses, then blinks 5 times again, repeatedly. This looks suspicious, but I don't know what it means.
The opener is a model 139.53674SRT1. It is a 1/2 hp Craftsman opener with belt drive and an outside keypad.
I would appreciate any help on this.
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