On the side of the motor unit you will find 2 controls. One is the down limit and the other is the up limit. I would take the cover off and look at what these controls are doing. The down limit is either mis-adjusted or broken. Turn the down limit in the opposite direction of the arrow printed on the cover to correct this if it is not broken. If you remember where the down force adjustment was I would put it back. It affects the safety.

This will happen when the lower bushing goes out on the gear and sprocket shaft. The limits skip timing and eventually the door doesn't know where it actually is during travel. A good Garage door company can fix this for you or you can order a gear and sprocket assembly with bushings from sears on line.

Here is a link that might be useful: Garage Doors & Openers

Garage door opener thinks its open. Take the chain back off the gear. Hit the opener and let it run til it stops. Gear should be turning clockwise. Put the chain back on and it should be fixed.

Thanks Doug, worked like a charm! Now I just need to figure out why the door keeps opening back up once it hits the ground.