Diagnosis
Remote control failure. Either the batteries in the remote are dead, the infrared sensor on the heater is blocked, or the remote/heater's receiver has malfunctioned.
How to Fix It
First, replace the remote's batteries (they're typically two AAA). Point the remote directly at the infrared receiver window on the heater's control panel from no more than 10 feet away. Make sure nothing is blocking the path. Test all the remote buttons. The heater should beep when it receives a valid signal.
If That Didn't Work
Try using a smartphone camera to test the remote. Point the remote at your phone's camera and press a button while looking at the screen. You should see a faint purple light flashing from the remote's LED if it's transmitting. If you see no light, the remote is dead. If you see light but the heater doesn't respond, the heater's infrared receiver is likely faulty.
Heads Up
The heater must be in standby mode (plugged in, main power light on) for the remote to work. It won't turn on a completely unplugged unit. Also, bright sunlight shining directly on the IR sensor can sometimes interfere with the signal.