If you have a grub prompt then you can boot your kernel manually. And I would say it is much more likely you have a malformed or missing grub.conf file rather than any corrupted mbr.
Assuming your kernel is in a /boot partition on /dev/hda1 you would do this:
grub> root (hd0,0)
Filesystem type is ext2fs, partition type 0x83
grub> kernel (hd0,0)/mykernel root=/dev/hda2 vga=791
[Linux-bzImage, setup=0xa00, size=0x10b949]
Remember that grub prompt has tab completion, so if you don't remember your kernel name:
Possible files are: mykernel myotherkernel etc.....
Once you are booted fix your grub conf.