Have taken the cover off of the unit & pinpointed the exact problem. There is a helical nylon gear that engages a worm gear on the electric motor output shaft. The worm on the motor shaft then turns the helical and operates the chain drive. The nylon helical has lost most / all of it's teeth. This would explain why, leading up to this total non-operation, sometimes it would operate partially, stop, then resume again if you kept using the remote. It was managing to find some still useful teeth on the helical. Visual inspection (and the nylon "shavings" in the cover and about) confirm my diagnosis. IF I could ONLY get someone who may have a manual for this, there MAY still be a new helical gear (and part #) available from Sears. Their "customer service" (by phone) say they no longer have records of my model (which is in fact a Chamberlain). This is NOT a Class 5 or higher gear by ANY means. There is nothing else wrong with this thing and why spend the $$ on a new opener
when all it needs is a stupid gear ???