My iMac refuses to start up. I get a white screen, several minutes to get the apple few minutes more to get the loading bar which only loads to 3/4 after many minutes then the screen goes black and I am back to a white empty screen.
iMac 27", macOS 10.13
Attempt resetting the NVRAM and also restarting in Safe Mode, if neither work, your next step is taking it to your local Apple Authorized Service Provider to be professionally diagnosed and repaired.
How to reset NVRAM on your Mac
Use safe mode to isolate issues with your Mac - Apple Support
iMac will not start
