One of our host servers has become unresponsive and crashed, resulting in a small number of servers being unavailable. We're currently investigating further.
UPDATE: The host has now been rebooted and is back online, affected guests have been booted back up and should now be available again. If you're still seeing problems with any of your servers then please contact support.
We identified an issue with the host configuration which may have been responsible for the crash, which has now been fixed.