Diagnosis
The wireless keypad has likely lost its pairing to the main opener unit. It can also happen if the keypad's internal battery is critically low.
How to Fix It
Start by replacing the keypad's battery (it takes a 9V). After putting in the new battery, you need to re-pair it. Go to your opener motor unit and press the orange 'Learn' button once. You'll hear a beep. Within 30 seconds, go to the keypad and enter your 4-digit PIN, then press the 'Enter' button. You should hear the opener click, confirming the pairing is restored.
If That Didn't Work
The keypad itself might have a fault. Try resetting it. With the battery out, hold down the 'Enter' button on the keypad for 15 seconds. Reinstall the battery and try the pairing process from the opener's 'Learn' button again.
Heads Up
If you have multiple keypads, you'll need to re-pair each one individually after a battery change. Also, make sure you're not accidentally using a PIN that was programmed for a different door if you have a multi-door setup.