You are stuck at grub prompt and not bash so some of the solutions that rely
on bash will not work. You have not even loaded the kernel yet.

I would seriously consider re-installing. BTW, did you do any drive
maintenance before you installed WUBI? You should check the drive for errors
and defragment it as a precaution. The way WUBI works is that it creates a
large image file for Ubuntu to run in and then mount this file as if it was
a drive or partition. If your drive is full, fragmented or has errors then
it will affect the performance or even corrupt the file. If you are going to
re-install then it is worth the extra step.

If you don't want to re-install then I would suggest that you look at other
options besides WUBI. This is a great technology, but it isn't bulletproof.
There are other ways to run Ubuntu with Windows such as dual booting,
andLinux, CoLinux and running it in a VM. If one method does not work others
might work better for you.