You put it on for to long? Your motherboard and display or the chain that connects them both is cut off in some way, since it didn't fall according to you keeping it on for too long makes OLED screens heat up and slowly burn away. You need to replace the one thing you bought the Steam Deck for which is the OLED. Should start working. Keep it up and it'll spread like a virus. Kinda already spread since you said the fan was acting weirdly. Better find a replacement for the screen now