Chosen Solution
This question is asked for one of my friends. He has a 2015 iMac 27" retina system, recently he got the screen replaced, however, when trying to power it on, he can hear the chime sound, but nothing showing on screen, screen is totally black, instead, the fan is very load. He tried to connect it with an external monitor, it is running and OK, but nothing displays on internal screen. He tried to do SMC reset, no help. Can any one advise what’s could be wrong here?
You stated the display was replaced, then what did get done? Was it working afterwards? If not then I would go back to the shop that fixed the system and either get it fixed again under their warranty or get your money back. It sounds like the backlight has failed. Using a flashlight held on the screen at about 20 degrees see if you can see the faint image of your desktop and its icons. If you do thats the issue if not the display it self is bad.
Personally I have repaired the logic board on one of these before, and the backlight is easily damaged. It is very possible that it is just a bad backlight or a broken backlight cable. @danj said, take it back to the repair store and ask for a refund and have then repair it properly or take it somewhere else that is more qualified.
If the fan is spinning loudly it may be the thermal sensor on the LCD screen on the rear was not replaced from the old unit. Without it, the CPU runs at 0.8ghz and the fan spins at full speed. As for the black screen, likely a cable was not properly connected. I’m thinking maybe the power connected was not seated well and if the thermal sensor WAS attached, it and the screen itself doesn’t receive any power hence, black screen, and full fan.
