My steam deck has a problem

Try holding a flashlight up to the screen at an angle in a darkened room and see if you can make out an image. If you can, that indicates the backlight has failed. Although there can be other reasons for a backlight to fail, chances are replacing the screen will bring your picture back.

If there's no image even with a flashlight, that's an argument that the LCD itself has gone bad.

Either way, the most likely fix for your issue is to replace the LCD. This guide will step you through the process.

Steam Deck Screen Replacement - iFixit Repair Guide

Good luck, and be sure to come back and let us know how it turns out. What you learn will help the next person who comes along with the same problem.