Sometimes it happens on reboot the system gets panic or some error message like "init not found" is displayed, preventing from proper booting; then as first step you can try pass init=/bin/bash (Grub boot loader) or linux init=/bin/bash (if Lilo boot loader). This will launch you straight into a Bash shell prompt. Then you can remount "/" file system and check /var/log/messages for any error. You can use the below command to remount "/" as read/write. Kernel panic could be because of a bad cpu or motherboard or bad memory.
#mount -o remount,rw /
If the above steps deos not work, then next option is to
1) Boot from the Linux First CD (boot CD).
2) Type "boot rescue" at Linux boot prompt.
3) After the bash shell prompt show up, type "chroot /mnt/sysimage".
a) Check for any disk error
#fsck -y /dev/sda2
check "fdisk -l /dev/hda" to check how many partitions you have
b) If you use Grub boot loader, type "/sbin/grub-install --root-directory=/boot". If you use Lilo boot loader, type "cd /etc; lilo".