This incident has been resolved. Affected VMs have either been started again (in the case of CPU VMs and shared servers) or shut down (in the case of GPU VMs). The latter can be started again as needed from https://xo.cl.cam.ac.uk .
The cause of this disruption was that the server on which VM disks and user data are stored failed to start with the correct network configuration after Sunday's planned shutdown (we suspect due to a bug causing it to make some incorrect changes to the configuration during boot). An attempt to rectify this problem 'live' caused about a minute's disruption, unfortunately just long enough to cause Linux VMs to time out I/O operations and shut down their filesystems.