Diagnosis
This is almost always a loose or failing connection between the main control board and the LED display panel, not a problem with the heater's function itself.
How to Fix It
Unplug the heater. Firmly tap the top control panel area with the heel of your hand. Sometimes a physical jolt can reseat a loose connection. Plug it back in and see if the display is normal. If not, try applying gentle, firm pressure with your fingers around the bezel of the display. Don't press on the screen itself, but on the plastic frame surrounding it. If the display flickers or becomes complete while you're pressing, you've confirmed a loose internal connection.
If That Didn't Work
There's no easy user fix for this. The display is connected via a thin ribbon cable that can work its way loose or fail. Repair requires disassembling the top control unit to access and reseat that cable, which involves removing hidden screws and plastic clips. For most users, as long as the buttons still work to control the heat and fan, you can live with a dim display. If it bothers you, it's a warranty or repair center issue.
Heads Up
Don't try to pry off the display cover or open the casing to fix this unless you're very comfortable with electronics repair. You risk damaging the ribbon cable permanently, breaking plastic tabs, and voiding any safety certifications. The heater will often work fine with a faulty display.