Richard> It's a 1TB SanDisk SSD Plus that I bought in July, 2018. 
Richard> It's the OS drive in a computer that runs 24/7, primarily as
Richard> a Plex server, but the media is on a different, spinning
Richard> drive.

That does sound like it died too early... do you have one of those
USB-to-SATA devices you could use to plug into another computer just
to see if it's alive?

Another option is to install Knoppix or some other bootable distro on
a thumb drive and boot there to see what you find.

As for the Windows and GRUB issue, I'm not expert enough to make it
work without some more details from you.  It might be enough to just
move the Windows boot drive to the same port that the SSD used to be
on.  Assuming it's a SATA port and not an M.2 port (which can be
SATA).

John