Diagnosis
The opener is in lockout mode. This happens after a power surge or if someone pressed the learn button for too long.
How to Fix It
Clear the opener's memory. Press and hold the learn button (the square, colored button near the hanging antenna) for at least 10 seconds. Hold it until the LED light next to it turns off completely. Release. Wait 30 seconds. The light should now be solid, not blinking. You can now pair remotes normally by pressing the learn button once and then pressing the button on your remote.
If That Didn't Work
The logic board may have a fault. Perform a full power cycle. Unplug the opener from the ceiling outlet or turn off the circuit breaker for 5 minutes. Plug it back in. The learn button light should be solid after the unit boots up.
Heads Up
Clearing the memory erases all previously paired remotes and keypads. You will have to reprogram every device.