Well there was a little power cut last night and the server rebooted. That's ok, it can cope with that. However I had fucked up my openvpn configuration, from months ago, and accidentally left my client connection details in the wrong place. On rebooting those client connections kicked in and caused the server to be unreachable. I didn't know that was the problem and have spent hours trying to figure it out. Problem is now identified and fixed.
Just another 'Go Fuck Yourself' from the universe :/