I got a old 2ds for 3$ with a friend, it was dirty so i cleaned it up and after putting everything back the screen stays black, theme music plays and games sound in the menu but it doesnt show anything, what can i do?

Put it back together the right way.
Sounds a bit rude, but… well… also sounds correct.Unlikely. 2|3 shut off when backlight isn't connected. Rather the video cable isn't correctly inserted.Maybe the backlights are malfunctioning.