Just because Windows won't start doesn't mean you can't do it! Obviously if the VM will not boot from the installed OS you either mount the virtual hard disk under the Host OS and check it that way or you boot the VM with a Live OS CD/DVD/ISO Image or USB via Plop Boot Manager. The Live OS OS CD/DVD/ISO Image or USB can be a Linux distro or a Windows in the form of WinPE or variant. Basically the same basic diagnostic/troubleshooting methods/methodsoligies that apply to am OS issue when run from a Physical Machine also apply to a Virtual Machine.
Otherwise restore the VM from a backup or if you haven't been keeping proper backups then delete the VM and create a new one if you don't want to attempt to repair it! 