Diagnosis
The keypad lost its memory. You need to re-sync it with the opener's main logic board, which involves a specific sequence at the motor unit.
How to Fix It
Here's the step-by-step. 1) At the motor unit, find the "Learn" button. It's a small, square, purple button on the back of the unit near where the antenna wire comes out. 2) Press and release the purple Learn button. The red LED next to it will light up solid for about 30 seconds. 3) Go to your keypad. Within those 30 seconds, enter your desired 4-digit PIN (like 1-2-3-4), then immediately press and hold the big "ENTER" button. You'll see the keypad light flash. 4) Test it by entering your new PIN and pressing ENTER. The opener should activate.
If That Didn't Work
If the red LED on the motor unit blinks when you press the Learn button instead of staying solid, the opener is in a different programming mode. Unplug the opener for 30 seconds, plug it back in, and start over from step 1.
Heads Up
This process erases ALL previously programmed remotes and keypads. After you do this, you'll need to reprogram any handheld remotes by pressing the Learn button and then the button on the remote.