Hi,
I'm having a similar issue with the graphics card no longer being detected. This has happened to me one more time a few months ago and card was visible after a restart.
No hardware changes were done inbetween these events and no major unraid version changes, only the plugin/driver updates.
nvidia-smi output from unraid console or from a container (plex) now shows "No devices were found"
In syslog (now and a few months ago) i can see this event:
Aug 10 10:31:32 Tower kernel: NVRM: GPU 0000:09:00.0: GPU has fallen off the bus.
Aug 10 10:31:32 Tower kernel: NVRM: A GPU crash dump has been created. If possible, please run
Aug 10 10:31:32 Tower kernel: NVRM: nvidia-bug-report.sh as root to collect this data before
Aug 10 10:31:32 Tower kernel: NVRM: the NVIDIA kernel module is unloaded.
Attaching the diagnostics file.
tower-diagnostics-20210810-1402.zip