Ctrl-C, kill -9 and server reboot fails means that there is an internal nvidia kernel error. Miner process gets stuck and the system can’t properly unload the nvidia driver during the system shutdown process... All the system gets locked, never complete the reboot process that’s why ssh never come back.

So... for the miner freeze issue, I suggest:
First of all try to execute a ”dmsg” command to get more info about your error (is always the same GPU ? PCI port ? Same kind of error ? ecc ...) then try to :

• change your PCI Risers (bad-quality ones can really become a pain in the ass)
• update your os kernel
• update your nvidia / CUDA drivers
• desable your overclocking setting and/or underclock your GPU

By experience, 99% of the rigs problems are generated by bad PCI Riser so try to always follow this hypothesis first.

Other tips: when your system is stuck like this, you can reboot without using physical power button using “reboot -q”

Hope this will help you

Try

-f, --force
    When used with halt, poweroff, reboot or kexec, execute the selected operation without shutting down all units. However, all processes will be killed forcibly and all file systems are unmounted or remounted read-only. This is hence a drastic but relatively safe option to request an immediate reboot. If --force is specified twice for these operations (with the exception of kexec), they will be executed immediately, without terminating any processes or unmounting any file systems. Warning: specifying --force twice with any of these operations might result in data loss. Note that when --force is specified twice the selected operation is executed by systemctl itself, and the system manager is not contacted. This means the command should succeed even when the system manager has crashed.

Also https://nico.schottelius.org/blog/reboot-linux-if-task-blocked-for-more-than-n-seconds/