I've Windows XP and Ubuntu 10.10 dual boot was working fine then suddenly while booting in GRUB if I select Ubuntu it shows me stupid Busybox error same as this and Windows XP is working fine The
askubuntu.com
BE CAREFUL....CHOOSE THE CORRECT PARTITION
(Personally I would use the livecd/usb stick)
My guess is that you have the filesystem corrupted or broken in any way, but it's easy to fix,
Everything can be done from the Ubuntu live CD. Boot from there, and
check which one is the partition where you have ubuntu installed on (sda1, sda5...). You can do it easily using Gparted. Even from there, you can right click on the partition and select "verify".
If you want to do it from the command line, just type:
fsck /dev/sda1 (being sda1 the partition where you have ubuntu installed).
And it will fix the ext4 filesystem problems. If you're getting too much questions while using it, add the "-a" switch to it:
fsck -a /dev/sda1
I have to tell you that use this with precaution. If the data is sensitive there, first make a backup if you can from the livecd to someplace else.
After the filesystem is fixed, you should be able to boot normally in ubuntu again.
Hope this helps you with that.
PS....if this a new install, you can always install again straight over the top of the current Kubuntu os....no need to format. It may be a good idea to verify the download of Kubuntu. Try the live cd/usb stick approach first