1. the back light may be bad

2. some buttons may be lowering your brightness ( this might be totally off base)

3. depending on your iphone OS version, you can create an shortcut called automation to periodically increase brightness to maximum

let us know. sometimes the Apple Store is better and sometimes not. your circumstances and wishes can be discussed ))

problem I had with my iPhone 7, but I went online to Apple.com and followed their easy instructions. Sent it in for repair of battery and it didn’t cost a lot. It’s worth a try. I’ll look for the correct link

FOUND IT!! 😊

https://support.apple.com/repair

Happened to me and they helped in 2 seconds. Just can’t remember what I did

Siri please brighten my screen

phrasing and attitude than that!

My 8 works fine. Crossing my fingers. Rarely, the gravity/rotation sensor locks up but a restart fixes it.

If your screen were completely black, it might be a loose wire. Doubt it. Maybe a "genius" can run diagnostics vis the lightning port.

Good luck! 📱