Originally posted by dalek
If you have no bootable kernel, you may have to boot from another distro and chroot in to compile another. You may still have a copy of the old kernel in /usr/src/linux.2.4<fill correct version here>/arch/i386/boot/bzImage. That should be the old 2.4.** version. That will let you boot and redo the 2.6.
Two other things you can do, if you've taken appropriate steps, are to boot from a backup boot floppy or you can often boot from the installation CD and use it as a rescue disk. Exact details depend on which distribution you're using.
Personally, I use many different distributions. When in a jam, I often find I can at least boot to some basic setup by using any of the boot floppies I've created, even if they're from another distro. I can also use other rescue CDs. Anything that's bootable can at least get me to the disk, then I can make whatever other adjustments may be necessary, then get back to the system I intend to work on. Worst case - install or reinstall the system in question, then recover from backups to get data back.
To install or reinstall GRUB to the Master Boot Record (MBR):
Boot the first install CD to rescue mode. This will mount your root
partition at /mnt/sysimage.
Type the following commands and press Enter after each command:
chroot /mnt/sysimage # (/ will now be /mnt/sysimage)