I don't know why it is happening, but I can tell you what is happening: When you boot your system, your computer passes control over to your bootloader (usually GRUB). Your bootloader loads the kernel from out of your /boot directory and also a temporary filesystem called an initramfs or initrd (also located in /boot). This initramfs gets a few more system things ready, and then passes off control to the real operating system (Ubuntu) by starting a script at /sbin/init. If /sbin/init doesn't exist (or some other problem occurs in the initramfs) then the kernel will just die ("panic") because there is nothing else for it to do.
So, some possibilities of what might have gone wrong:
* The hard drive (or hard drive partition) that contains your /sbin/init crashed or its data was corrupted (recent power outage? old hard drive...?)
* The latest initramfs/initrd you got (say from a software update) is corrupted somehow or not configured correctly. When you boot, and the bootloader appears, is there a different kernel version available for you to try instead of the default one?
* At some point you accidentally deleted your /sbin/init. Unlikely, but dumber things have happened.
Last edited by hydraMax; 01-23-2012 at 11:34 PM.