Diagnosis
The backlight LED for the LCD is failing or the ambient light sensor is stuck in a low-brightness mode.
How to Fix It
First, manually override the auto-brightness. Go to Menu > Preferences > Display. Set 'Backlight' to HIGH and 'Auto Dim' to OFF. The screen should immediately brighten.
If That Didn't Work
The internal LED may be failing. This requires component-level repair or unit replacement. As a temporary check, shine a bright flashlight directly at the screen. If you can suddenly read it clearly, the backlight has failed.
Heads Up
A persistently dim screen can be an early sign of a failing power connection, often related to the C wire. Verify your wiring before assuming a screen defect.