I have a Windows 8 laptop. I ran Ubuntu through a live USB and installed it (secure boot doesn't let WUBI otherwise).
When I used boot repair to boot back into Windows, it worked. I restarted to go back to Ubuntu, but it showed this:
error: unknown filesystem. grub rescue>
No matter what I try from other threads I cannot go back to Windows 8 or Ubuntu. I even put the USB back in, but nothing happened.