I had a cluster of proxmox servers (debian) that refused to reboot. I had just run some updates as part of the monthly maintenance schedule and issued the “reboot” command. I have molly-guard installed, so I had to type the name of the server as well, but they were just sitting there.

Checked the /var/log/messages for any ideas – system preparing for reboot.

The server continued to function as it had before the reboot command – I could use apache, ssh, webmin, etc all fine. As the servers are located at a not-close datacenter AND I have KVM control from any VPN connection, I decided to try to find a way to force it down. Unfortunately our DC does not allow us to control the smart PDUs installed in each rack as they retain the right to remotely monitor our power consumption.

After a short search on my favorite g search engine site, I come across this:
echo 1 > /proc/sys/kernel/sysrq
echo b > /proc/sysrq-trigger

As soon as you hit enter on the second command the server will drop and start to reboot.