The virtual machine host "hummingbird" has failed. There was no apparent reason for this failure. We're sending an engineer over to the datacentre for a closer inspection.
I'll post here when we have more information.
The machine has now booted, and is currently checking its discs.
I'll post more as I have it.
The disc check is about 50% of the way through now.
The machine is back up now, and VMs should be beginning to start up one by one.