screen suddenly became very dark

[AC Question]

sound and everything else still works

 

[belodion]

The screen may have failed, or be close to failing.
But try Safe Mode, in case some app is interfering with screen brightness.

 

[Rukbat]

If it's not an app doing it, it's probably the LED backlight (at the rear of the screen) or something on the motherboard controlling screen brightness that failed. In either case it'll have to go into the repair shop.