Bootcamp Windows 10 Fails to Start After Interrupted macOS Update
A workaround I have discovered is to go to System Preferences -> Startup Disk, and select the OS that you cannot now access from the boot screen as your default startup OS (in my case Boot Camp Windows 10). Then at the boot screen you can access Mojave as normal when you want to. It is not really a fix but an easy workaround until the fix comes out. It worked for me at least.