Well, this is interesting. I have three systems, all of which now have the same problem.
I was running "yum update" on these machines via a vnc connection (running a vnc desktop on one of them, and logging into the others with a a gnome-terminal on my vnc desktop), when my vnc desktop suddenly "went away" for some reason. And that killed the "yum update" jobs on the computers.
Subsequent to that, I logged back into the machines and ran yum update again. It told me to run yum-complete-transaction. When I ran yum-complete-transaction I got screen after screen of "x is a duplicate with x" where x consists of a huge list of packages.
I then ran "package-cleanup --cleandupes" and then ran "yum update" again and all appeared to be well. "yum update" completed without error and I thought I was home free.
I then rebooted the machines and found out that I'm still out of luck. After the initial grub screen I get this:
Kernel panic - not syncing: VFS: unable to mount root fs on unknown-block(0,0) PID: 1,comm: swapper not tainted 2.6.32-358.0.1.el6.i686 #1 Call trace
Followed by a series of numbers that I can post if they're needed.
I booted one of these machines off of a Centos 6.4 "minimal" CD and ran the rescue mode. It mounted the drive under /mnt/sysimage with no problem. I can see everything on it that I expect to see.
I then booted the CD again and tried running the "upgrade an existing system" option, and told it to reinstall the bootloader. That's about all that it appeared to do: "Installing bootloader", then it told me to reboot. Which I did.
And I got the same kernel panic again that I just posted above.
What has gone wrong here and how can I fix it? All of the data seems to be on the drive just like it should be, but it won't boot up.
Again, I have three systems that appear to have exactly the same problem.