I have an iMac 2011 which was running High Sierra well. But yesterday the screen suddenly went white. Now in reboot the screen goes grey with the logo and goes through the status bar but then goes to a complete grey screen and won’t load. I’ve tried resetting NVRam and SMC and other usual steps. When I hit option I get HD, HD recovery and my Time machine but with each option I select it just goes to the grey screen. I got into safe mode once and scanned utility desk but nothing. However the graphics were loading slow. Now I can’t get safe mode anymore. Any suggestions? Is it hardware or software? I am panicking here!