Have you looked at the breakdown of your iMac at iFixit: The Free Repair Manual Where is the backlight circuit located and is it on a separate board? The backlight circuitry might be located within the display assembly.
as I can see its an LED driver board on the display in this iMac,
try following this guide iMac Intel 27" EMC 2390 LED Driver Board Replacement - iFixit Repair Guide
see how you go it could be that the display itself is going bad maybe some of the led array
do you have any bright or darker spots on the screen when it does come on ?
also it could be the GPU going bad as ive seen this happen then also.
No the screen looks fine just turns off. I have it displaying to a external monitor right now I don’t know if the rules out the gpu.