told the gpu had died.so i fitted a new one same...stock for 2011.. 27"..
still nothing..can i plug a second screen in to see if that works..if so how....got the thunderbolt to thunderbolt cable...pressed command..f2....nothing...thanks
1. Boot machine hold down Option key. Keep holding.
2. Confirm there is the Chime. If no Chime, your problem is PSU or GPU or logic board.
3. Wait another 20 seconds, then let go of the option key.
4. Take a strong torch light and shine onto the screen. Try to identify some icons around the center.
5. If you are able to see anything on the screen, your problem is neither GPU nor screen. The problem is related to backlight. This could be PSU, backlight circuit, cable connection or screen.
If you see nothing, your problem can be GPU or screen.
If you already tried another GPU, you could try external screen. Connect any screen eg. via mini Displayport to HDMI to a HDMI screen.
Alternatively, remove the screen, and observe the LEDs. Let us know what you see.
6. Make a boot stick of High Sierra, or use "Internet recovery". Read up on how to do this. Easy. You can install MacOS onto a thumbdrive, you should have one. Then at least you have a bootable system.
