Chosen Solution
I have a late 2014 iMac 27 4.0Ghz i7 32Gb Ram 3Tb fusion drive and a Radeon R9M295X 4Gb Graphics, The issue is no image displayed, Backlight working (brightens and dims normally) when plugging in an external monitor will display before login so no login issue and all hardware / software working corrrectly on external display. If I go to the display settings both onboard display and external display are recognised. It started after an automatic update overnight (Some time ago tried updates a couple of times since in case it was a software issue), I was working on it fine one day and put it to sleep the next day I have the above issue. The iMac I have owned from new and has sat on my desk not been knocked / dropped etc, also has never been opened any ideas?
Not good ;-{ You have a very sick logic board which will take someone with deeper skills to diagnose what component failed and if it can be repaired, your other option is just replace the logic board.
