Using Ubuntu in a VM on a Windows host is not the best arrangement, VMs
create large files that can become corrupted by the Windows file system
which is more susceptible to fragmentation, crashes, viruses and damage.
Keeping important data on a VM is not advisable unless you back it up. It is
much more secure to create a Windows VM in an Ubuntu host because the Linux
file system does not fragment, there are no virus threats and crashes are
fewer and usually don't produce corruption.

If the VM is not damaged then you can probably restore it. VMs can be moved
from one drive to another and even to a different computer, provided you
have a large capacity usb key or drive. If you have lots of available HD
space you can try to create a second instance of the same VM. Just give it a
new name. To do this you create a new VM, but when it comes to choosing the
drive, use the old VM. If the VM is damaged then you are probably out of
luck.

If you do manage to get the VM going, save your data to a more secure
location and make a backup of the VM.