Comment 2 for bug 709952

Please supply more details about the error message. The message "The root.disk does not exist" is clearly incorrect, because you can still boot the older kernel. Is dropping you at a busybox prompt? This has been a fairly common problem with the post-install procedures resulting in a bad initrd.img - and I'd guess this is your problem.

In which case you can try regenerating it - boot with the -22 kernel and run:
sudo update-initramfs -k 2.6.35-25-generic -c