Diagnosis
Either the remote's batteries are dead, the IR sensor on the AC is blocked, or the remote itself has failed. It's almost always the first one.
How to Fix It
Replace the remote's batteries with fresh ones. Make sure they're inserted with the correct polarity (+/-). Point the remote directly at the infrared receiver on the AC's control panel (usually a small, dark plastic window near the display) from no more than 10 feet away. Check that nothing is physically blocking the sensor.
If That Didn't Work
Test the remote's IR emitter. Use your phone's camera (the main lens, not the selfie cam). Point the remote at the camera lens and press a button while looking at your phone screen. You should see a faint purple or white light flashing from the tip of the remote on your screen. If you don't see it, the remote is dead.
Heads Up
Sunlight or very bright ambient light can flood the IR sensor on the AC and make it unresponsive to the remote. Try shading the sensor with your hand when you press the button.