I'm not familiar with the AllStar brand but the first place to start when troubleshooting any garage door opener is to disconnect the opener from the door using the pull rope. Then ensure the door is functioning properly. It should open and close smoothly and easily and remain balanced at about chest height. If the door isn't working right, those issues need to be fixed because the opener won't be able to open the door or it may be damaged. With the opener still disconnected, operate the opener. If it doesn't move, then something internal is broken or out of adjustment. With Chamblerlain (including LiftMaster and Craftsman) openers there is a plastic gear that is a common source of trouble. It might not be a bad idea to unplug the opener for a few minutes to reset any electronics too.

Thanks. Even with the door disconnected, there is no movement of the belt.

My next step would be to remove the casing around the drive unit. If the plastic (neoprene) gears are not obviously worn, and nothing else 'looks wrong', I'd trigger the GDO again to see if anything is turning.

BTW, when released from the GDO, does the door stay about 1/2 open when raised to that position?