richard

I have just purchased an iMac 27 inch late 2012 but what I can't work out if it's the graphics GPU or the screen...

It's really simple there is nothing on the drive. It will be black. Before you do anything you have to do either Internet recovery right from start-up or it will always just display a black screen. You need at least a USB bootable drive or you could try an Internet recovery but obviously the Internet recovery takes a long time as it has to download everything and the basic system tools. I'll try the Internet recovery even though it takes a long time the guide can be followed here. Once you get into your basic tools you need to use Disk Utility and make a petition and format it. Once you've done that your exit disk utility and then proceed with the install of whatever OS it is. It's basically looking for something that isn't there so I can't show anything. https://support.apple.com/en-gb/HT201314 Here is how to use MacOS Internet Recovery: (1) Shut down your Mac. (2) Hold down Command-Option/Alt-R and press the Power button. ... (3) Hold down those keys until you a spinning globe and the message "Starting...