If the door is up and will not close, one possibility is the garage floor is reflecting light that interferes with the light beam at the bottom of the garage door opening.
The light beam can be overridden by holding down the button in the garage until the door is fully closed (this will not work with a remote or the keypad outside the garage). If that works, the problem is almost certainly the light beam.
Try changing the angle on the light beam to point slightly up and back toward the inside of the garage.
One side of the two light beam components has a green light and the other side has a red light. The green light should be on steady. This indicates the light beam is being sent. The red light is on steady when there is no obstruction and flashes when there is an obstruction (2 flases, pause, repeat).