Repair garage door chain

Do you have a button switch on the wall to open - close? There are usually two wires going to that. I don't think it will hurt to short those wires momentarily to test it. If it did, Lift Master would get a lot of returns because I'm certain many people have shorted the wired pair upon installation. My Craftsman has two wires going to a multiple switch panel. I can open - close the door, turn on-off the opener light, and lock the remote controls. These functions are controlled by adding a resistance to the wired pair. each control has a different resistance. The open-close is just a short. the unit remembers what direction it went the time before. IE: if you short once to open the door, then short again will close the door.

It sounds like the chain might not be on the sprocket that drives it. Get up on a ladder and look at the top of the unit. You should be able to easily see if the chain is off the sprocket. Try running the opener while you are up there too.

If the chain has come off you will need to loosen the chain tension, place the chain back over the sprocket and tighten it to the correct tension. Your user manual should have details on how to perform this but usually there is a screw you adjust on the trolley that controls the tension. Make sure to disconnect your door from the trolley before you do this.

Here's YouTube video that shows the process.