My door starts to go down but then reverses.

This is a very common occurrence. Most of the time this is a result of a misaligned set of safety photo eyes on the bottom of your vertical garage door tracks. These are often bumped out of alignment from objects like a trash can. If your operator light is flashing after the door reverses, this indicates that is your photo eyes. Try re-aligning those first. Both eyes should be lit up which means they are both in sync.