Diagnosis
The remote's infrared signal isn't reaching the unit's sensor, or the remote itself has failed.
How to Fix It
First, check the IR sensor window on the front of the AC unit. Is it blocked by dust or an object? Wipe it clean. Then, stand directly in front of the unit, 5-10 feet away, and point the remote straight at the sensor. Test it. Other IR devices (like a phone camera) can see the remote's LED glow if it's working.
If That Didn't Work
The remote circuit board may have a bad connection. Try removing the batteries, pressing and holding any button on the remote for 30 seconds to discharge it, then reinsert batteries. If still dead, you need a replacement remote.
Heads Up
The unit's control panel has all the same functions as the remote. You can operate it manually while you sort out the remote issue.