This little code snip can help you out in a pinch.
Try this if you have an unstable server with running guests and reboot is not an option.

service pve-cluster restart && service pvedaemon restart && service pvestatd restart && service pveproxy restart

If you do it in the shell window, it will disconnect the shell, but seems to complete the command just fine.
You should see the nodes briefly grey out and come back.

I've read that you need to reset all of those, in that order. See the linked wiki article for more info.