After a power outage - yes another one! - a VM would not start - checked the boot and the VM had an ISO in the DVD image that was no longer in place. Removed ISO from DVD boot-up and all was well..
Log Name: Microsoft-Windows-Hyper-V-Worker-Admin
Date: 28/11/2017 22:00:05
Event ID: 12030
Task Category: None
User: NT VIRTUAL MACHINE\
'AWCNSAN' failed to start. (Virtual machine ID )