It appears that a regular restart of the server caused it to try and apply microsoft security updates.

For some reason this failed and the server stalled.  We have now managed to get the server to restart correctly and it is currently starting up applying updates (23% and progressing slowly).

Assuming this completes successfully it will then take a few additional minutes for the individual VMs to restart.

We will continue to monitor the restart.  Apologies for any inconvenience.