We have some software that runs on that VM (the big one is Quickbooks) and that software can still be accessed and runs fine. We tried turning the VM off from the Hyper-V Manager, and then turning it back on. We see the windows startup screen in the open Hyper-V window, but then it goes back to the empty black screen.
I've worked out it was because my monitor was turned off on my host machine, when I got home and turned it back on everything worked fine. I guess this has something to do with my graphics card going to sleep when my monitor is off or something but I'll just leave my monitor on all the time as a work around for now.