I have proxmox 8.1 running on a legacy HP Z400 workstation. It is booting to BIOS the box does not support HEFI. It has been working fine with no issues until I installed the upgrades last night. Now it boots to Grub Rescue.
I have tried multiple ideas from the Proxmox site with no success. When I boot using the debug I can run a blkid command and I can see the disks. That is as far as I have gotten.
I think my issue is unique because all the troubleshooting tips are for UEFI enabled boxes.
Unfortunately it is really hard to give spesific and targeted advice for such issues without knowing a bit more.
My best advice would be to fire up a live usb stick with your favourite distribution and see if you can learn more about the problem that way. There is also a tool called boot repair that can fix some grub-issues.
I had a similar issue with my Ubuntu server a few weeks ago and wrote a post about my approach to get the server going again. I’ll leave it here so hopefully it can give you some inspiration for troubleshooting and paths to research or ask more about if you need help.