Safety: Garage doors are heavy and spring/cable systems can be under significant tension. Do not loosen or disassemble high-tension hardware unless you are qualified to do so.
Check the Opening
Remove obvious obstructions from the doorway and sensor path. Do not disable safety devices to force the door closed.
Watch How the Door Moves
A door that starts down and reverses is a different symptom from a door that never starts moving.
Crooked or Binding Door
If one side is lower than the other or the door scrapes heavily, stop operation and have the mechanical system checked.