If for some/many reasons after a kernel upgrade, the system does not reboot. Some hosting control panels can reboot the server into rescue mode. In this mode, you can edit the lilo.conf to go back to the previous working kernel. Make sure you backup the file first.
# mount partitions to fix lilo.cnf
mount -t ext3 /dev/sda3 /mnt
mount -t ext3 /dev/sda1 /mnt/boot
# edit the file to boot to the previous kernel
# and run lilo to activate the boot sector on harddrive
# should not see any fatal error
lilo -v -v
Also, if you use “yum update” to upgrade the kernel, the default kernel might have too many extra/redundant modules that might cause the system hang while booting up. If it is the case, use a modified kernel (compile your own) with some minimal modules enabled (can see the modules in /lib/modules/126.96.36.199, change the version if needed).
The files in /boot