Diagnosis
Either the remote's IR signal is blocked, the sensor on the AC is obstructed, or the remote itself has failed (less common).
How to Fix It
First, clean the IR sensor window on the AC unit (a small, dark plastic lens on the control panel) with a dry cloth. Ensure no objects are in front of it. Stand within 10 feet and point the remote directly at the sensor. Test by covering the remote's IR LED with your phone camera—you should see a faint purple light when pressing buttons.
If That Didn't Work
Perform a hard reset on the AC. Unplug it from the wall for 5 full minutes, then plug it back in. This can clear any glitch in the main control board that might be ignoring the remote signal.
Heads Up
The remote uses a common IR frequency. If you have other strong IR sources (like certain LED lights or a sunbeam hitting the sensor), it can interfere. Also, after changing batteries, press a button firmly for 3 seconds to wake the remote up.