I'm a pretty new Ubuntu user, and I can't figure out what to do from searching other threads.

Anyway, here's my situation:
I had Ubuntu 12.04 installed on a partition, and OS X 10.7 on another, and it worked great. Then after I installed 10.8, my Ubuntu partition no longer worked. It gave me:
"error-unknown filesystem

After doing some reading, I tried to repair grub by booting from a Live USB disk. But now I'm getting the same "grub rescue" prompt when I try to boot from the live usb!

Any ideas?

Thanks a lot