There are times that your sysetm is not stable and you can not reboot it, if you have access to console:
Alt+SysRq+u will force unmount all filesystems
Alt+SysRq+i will force kill all tasks
Alt+SysRq+b will force reboot
Alt+SysRq+o will force poweroff
If you are on a ssh shell you can not issue these commands and should do this:
enable feature:
echo 1 > /proc/sys/kernel/sysrq
issue a command (reboot for example):
echo b > /proc/sysrq-trigger
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