Diagnosis
The travel limits have reset. The opener no longer knows how far the door should travel, so it hits the default safety reversal.
How to Fix It
Retrain the travel limits. Press and hold the black 'Learn' button for 10 seconds until the red light goes out. Press your remote 'Up'. The door will move. Let it go all the way up and stop on its own. Press 'Up' again. It goes down. Let it touch the floor and stop. Training is complete.
If That Didn't Work
A sticking roller or binding track can make the opener think it hit an obstacle. With the door disconnected, manually run the door up and down the full track. It must be smooth.
Heads Up
Do not try to adjust the physical limit screws on the side of the motor. The RJO101 has electronic limits set by the training sequence. Turning those screws will cause damage.