So unlike a lot of the other posts that I've seen I'm running into a problem with my Debian dual boot installation after I tried to upgrade.
Background: dual boot has been working for years with grub boot loader being the default boot loader. Yesterday I attempted an
apt-get upgrade and
apt-get full-upgrade in an attempt to upgrade my installation from Debian 9 (Stretch) to Debian 10 (Buster). Finally I ran
apt-get --purge autoremove.
I rebooted the system and the partition doesn't appear as bootable anymore in the BIOS.
What I've Tried:: I figured it was an issue with the upgrade breaking something in GRUB so I installed a Debian 9 ISO to a USB and attempted to install GRUB through the rescue mode. This always fails. Note: I used the graphical interface.
I Tried to use the rEFInd Boot Manager, but the only option that gave was "default" and it just rebooted my machine without any noticeable difference.
I confirmed that the partition I was trying to re-install GRUB on was the debian partition because I was able to open a shell into /root on that partition.
Punchline: at this point I'm at a loss for how to get this partition bootable again. Worst case I can sync the whole partition externally and fresh reinstall Debian but I'd prefer not to do that.
Also let me know what information is most useful for y'all to help me out here.
SOLVED: Did what Jonah mentioned below and solved boot variable errors with the link I shared in subsequent comments.