I have always double booted... so.. I never came to find out if you need to have GRUB in a single linux system. Someone else might be able to answer that one.
You can install GRUB in the MBR or in the 'boot sector' , I would say that you need to reinstall your GRUB fromt he rescue disk.
From the man pages:
To use GRUB, you need to install it on your drive. There are two
ways of doing that - either using the utility `grub-install' (*note
on a UNIX-like OS, or by using the native
Stage 2. These are quite similar, however, the utility might probe a
wrong BIOS drive, so better be careful.
I guess you cought one of this '..probe a wrong BIOS...'
So... rescue disk and reinstall from there :P