My garage door goes up about 6″ and comes back down?

This is a symptom of a broken torsion spring. The torsion spring acts as a counter-balance for the door. With a broken spring, the door is essentially dead weight. A heavy, unbalanced door is detrimental to the operator and can cause undue stress on the entire system.

My door goes down about 6″ and comes back up?

This symptom is usually associated with the operator. Misaligned safety sensors, mal-adjusted force settings, and an unbalanced door can all contribute to this problem.

The cable is either broken, or it has too much slack?
Usually, a broken or slacked cable is indicative of a broken spring, on a single spring system, or a “crashed door,” or a door that has come off track.
My motor sounds like it’s running, but the door isn’t moving?
Most operators will last somewhere between a decade or two, but there are a number of components that can fail. Whether it be a motor, gear and sprocket, capacitor, limit switch, etc…any of these parts can fail, leaving the opener virtually useless. A repair can often remedy the situation, but replacement is usually the most economical approach.