I suggest, as a first step, that you boot a liveCD. You Ubuntu install disk may also run as a liveCD. Then, run fdisk on your Ubuntu partition to see if fdisk can find and repair a damaged system.
Do Not Mount Your Ubuntu Partition. Fdisk must be run on an unmounted partition.
If you installed Ubuntu with ext3 or ext4 filesystem (both are journaled filesystems) then fdisk should be able to recover the journal to make repairs, if I understand the workings of fdisk correctly.
After running fdisk, try a normal boot to see if it worked.
Last edited by bigrigdriver; 05-11-2010 at 08:13 PM.