Dead backlight on iMac, LCD and all else OK
Try to reduce the screen brightness to minimum. Put the computer to sleep and wake it up. 

Better (90%)?

--> e.g. https://www.youtube.com/watch?v=gSzmyqQFZ2M

Not better (10%) -->

Toast your graphics board in the oven

e.g. https://macman860.wordpress.com/2013/02/03/baking-a-graphics-card/

Better (90%) --> You're welcome

Not better --> New inverter board