Diagnosis
The infrared (IR) receiver sensor on the fan's control panel is either blocked, faulty, or has become desensitized. It's a little dark plastic window. Sometimes a direct blast of sunlight can also temporarily blind it.
How to Fix It
First, check the obvious. Make sure you're pointing the remote directly at the sensor on the fan (not the display) from within about 15 feet, with no major obstacles. Try the remote in a dimmer room to rule out IR interference from bright lights. If that fails, the issue is likely on the fan's end. Unplug the fan for a full 5 minutes. This resets the main control board and can sometimes wake up a dormant IR receiver circuit.
If That Didn't Work
If a hard reset doesn't bring the remote function back, the IR receiver component itself has probably failed. It's soldered to the main control board inside the base. This isn't a user-serviceable fix unless you're very handy with a soldering iron and can source the exact component.
Heads Up
The remote uses a standard 3V CR2025 coin cell battery. Double-check the polarity when you insert it; putting it in backwards is an easy mistake and won't damage it, but it won't work. The positive (+) side should face up, towards the buttons.