My Fedora 15 installation just failed to boot because I tried to use a software mirror on my boot partition. (Apparently GRUB 0.97 doesn't "understand" software mirrors with superblocks using anything after 0.96.) The symptom was that GRUB went into the GRUB command line mode, which is what the OP maker reported. (I solved the problem by using the GRUB command line to install GRUB on a /boot directory under my root directory - which was not in a logical volume - and then editing /etc/fstab to use the new structure. [Of course, I had a backup copy of my /boot partition that I could copy into the new /boot directory.] But this procedure is not something that I would try to walk a new user through, especially via comments here, and most new users don't have backups.)
So, the problem may simply be that GRUB is not able to read his /boot partition. Perhaps the OPer failed to create a separate /boot partition, or somehow created it inside a logical volume. That should be easy to check from a live CD, but I'd really suggest a fresh install of Fedora 15 from an up-to-date installation disk. And, as I said, not using the default "logical volume" scheme.