I looked through any recent updates to the machine and saw nothing likely, googling only gave me my tweet for help(!) so I hurriedly connected the backup drive, did a machine backup - I was glad I already had root access and then tried a reboot. This failed to work - wouldn't shutdown with a message:
could not log bootup - already in useso a magic key shutdown (using the sysRq key) was resorted to.
A few deep breaths later and the system rebooted and I logged in - then I realised the consequence of the ssh crash - one of the system scripts which run when you open up a terminal checks whether the machine is running ssh and, if so, asks you to unlock the key - for passwordless logins - if ssh is confused - as it evidently was - maybe that script was hanging there.
So I guess killing and restarting sshd would have avoided the reboot as to why ssh was broken that's another issue!
At least the server had been up for a month so it wasn't too unstable!