There should have been a more informative error message a few lines prior to the one you reported, which is the symptom, not the cause.
Since yum only installs the newer kernel, your older kernel should still be available, and you should be able to boot it and look in /var/log/ to see if there were some messages posted.
Also, if you're using GRUB to boot, use GRUB to edit the new kernel= line to remove the quiet at the end of it, which will give you more detailed messages during the boot. (Editing the boot instruction in GRUB only changes the copy in memory, not the actual boot file.)
The actual cause of the boot failure should be several lines before the "Kernel panic" line, and is usually some disk problem, of failure to activate the correct logical volume.
Last edited by PTrenholme; 03-11-2006 at 10:37 AM.