Screen black not working

I have an old iMac I believe it’s a 4K one and one day it died the screen complete went quits and to this day I believe it might be a backlight issue because when I shine my phone light on it I can see everything working but there is no backlighting if anyone nows how to replace the backlight please help! I’m not sure if I need to replace the screen or what all I know is I want my Mac Back!