This thread was a bit confusing because I was not sure whether the issue was hardware or software related. For anyone with a similar issue, I'll try to summarize here.
1. On my multiboot machine one linux OS, LMDE-3 became unresponsive. I was never able to figure out why. Researching the logs would have been useful.
2. Being impatient, I did a hard shutdown...almost surely a mistake which probably led to corruption on my SSD as @all41 pointed out. He mentioned that there are other ways to make an emergency shutdown. Also doing an ssh from another machine on the network will often work. But I was impatient.
3. Corruption included changing the SSD from a GPT to an MBR type, the loss of the LMDE OS, and corruption of GRUB. I was able to restore the SSD type back from MBR to GPT without losing data (my other OSes) using gdisk via this solution
using the "w" option. In retrospect, wiping the SSD completely and re-partitioning would not have been much more difficult considering that I was installing new LTS versions of my OSes.
4. The corruption of GRUB put me into GRUB Rescue > land...a hellish place to be. This would have been easier to resolve if GRUB's /boot/efi was on the same SSD as my linux OSes something @all41 encouraged.
5. @cretsiah made a post suggesting that BIOS settings can be an issue; namely whether the machine boots via UEFI (only) or either Legacy (MBR) or UEFI. This led to some confusion on my part: my original setting was the dual style (either, depending), but I changed it to only UEFI. This would have been OK, but once in the BIOS, one is tempted to do other things and I did; changing the boot order etc...It's easy to mess up, so my conclusion is when in the BIOS, be very careful.