Today I installed macOS High Sierra after doing some research. Obviously it was a big mistake. I used it successfully for about an hour, then when I was online I unplugged my laptop and everything went kablooey. Like a blue screen for Macs. Staticky horizontal lines with constant flashing - that was all I saw.
Made a little progress when I restarted holding down commandR. But the screen was split horizontally across the middle, always moving, so I could see the top choice in utilities which was recover but I couldn’t choose it because I couldn’t get to the button on the bottom. It was in the bottom half of the screen that was just lines.
Eventually the Utilities screen would just slowly fade out so I couldn’t see it anymore. So I turned the machine off again. Also tried restarting my router because I read that errors were sometimes with Wi-Fi. No luck.
I WAS FINALLY ABLE TO FIX THIS! I FOUND A KEYSTROKE COMBO ONLINE THAT ALLOWED ME TO HAVE THE BACKUP DRIVE (WHICH I'D USED JUST BEFORE THE FATAL UPGRADE) DO A RECOVERY. I'M BACK TO YOSEMITE, AND VERY HAPPY!