This iMac is my old war horse. Core i7 3.4, 20GB RAM, High Sierra. 500GB Samsung ssd and also has the original 1TB hdd inside. The ssd is adequate for all my needs so the original 1TB hdd is for backup purposes. I also have a CCC clone backup as well as Time Mach backup. I did the install of the ssd myself 2 years ago and all has been excellent with the puter until last night. I was reading news on the internet and out of nowhere I heard a pop (or snap), and the screen went black though I could see a bit of light around around the edges telling me that the computer was still running. I couldn't do anything with it so I shut it down and restarted.The restart began as normal showing the status bar letting me know that the SSD was loading. About three quarters of the way across, the status bar stopped and the screen went completely to white. I restarted holding down the option key.The icon of the SSD and also the icon of the internal 1 TB drive both showed. I selected the SSD icon and hit return. I got the same situation again where the status bar stopped halfway across and then the screen went to white. If I continued to let the computer run, it went into a loop of the same thing I have described happening over and over again. I shut it down and restarted again holding down the option key. This time when the drive icons came up, I selected the internal 1TB drive one of whose 3 partitions contained a CCC clone of the ssd but the whole process started over again with no improvement. Later, I tried connecting an external hdd that had another CCC clone on it and while the icon of the external showed up while holding down option, the same thing happened as happened all the other times before. I have tried starting up in safe mode but nothing I do will allow me to somehow get to disk utility and possibly run first aid. I also tried booting from a High Sierra install flash drive - no luck. The computer doesn't shut down unless I hold the power button down. If I do not shut it down that way.The top of the case feels extremely hot after about 10 minutes. Last thing to tell you (and maybe I should have mentioned it first), I had been using a vcr recorder connected to a Vidbox (analog to digital converter) and it was connected to a USB port on the computer. Although I was not converting any vhs tapes to digital at the time the computer made the pop noise, the Vidbox was receiving its power via the USB port. Earlier in the day and also the day before I had done some vhs to digital conversions without any problem. So I'd be glad for advice or opinions before I call Apple or otherwise try getting the problem diagnosed. Lastly, sorry for the length of this post. Wanted to give you something to go on. I've been using Apple computers since the IIe and luckily I've never had this happen before. Many thanks for your thoughts. Reed.