They usually blink going down because it's detecting something is in the way. The other thing to check is to make sure both "eye" units at the bottom are facing each other and have not got knocked off their holder or something is covering one of them.

If it's a Craftsman and the fault is at the sensors the light should blink 10 times. (not sure about the other brands). How are you closing the door now? If you need to press and hold the wall button until the door is fully closed, then that is another sign that the fault is with the sensors "eyes" at the bottom of the door. The sensors should both be lit with the lights on steady. No flickering or intermittent connection.