kernel panic - unable to mount root fs

I have a Ubuntu 10.04 desktop and one day it just refuses booting up. Here are my steps to get it going again. Please drop a comment if it helped you.

- press escape when grub is load. Choose "recovery mode" to see where in the boot process it failed. In this case it is trying to mount root and failed.
- rebooted the computer from the bootable CDrom. Chose "fix a harddisk"
- once the initial setup is completed, try to mount the harddisk as root. For me it worked.

At this point I have 3 suspects:
- grub is corrupt. I tried reinstalling grub from the CDrom and that didn't help. Moreover I have been able to change options, tried older kernels, so not likely.
- blkid or device id somehow changed. I double checked the blkid values and devices with both the grub menu and /etc/fstab and they haven't changed.
- it took a while for me but I finally did a ls -l on /boot. All the files except one was dated one day before the failure. It was also odd that there was no backup (.bak) of the file. The file was initrd-xxx. I couldn't find the documentation on how to roll back the file, so I grabbed the same file from another Ubuntu box, copied it over. Rebooted and it started working again.

I googled a lot during this process and there was nothing really helpful. My suspicion is that there was a kernel update and somewhere in the middle of the process it failed.