Expanding on the previous reply by drs305, here is a detailed step by step description that will help you resolve the issue right away.
After boot we get the prompt:
error: file not found.
The grub information on disk has been corrupted and the system can not boot from the proper partition
1. Boot with Ubuntu Live CD
2. Open terminal (command prompt)
You will get a list of partitions, similar to the following list:
Ubuntu partition is the one with the name "Linux" (not necessarily the one with the star, although could be).
/dev/sda1 13 102400 de Dell Utility
/dev/sda2 * 13 1926 15360000 7 HPFS/NTFS
/dev/sda3 1926 30892 232676566 7 HPFS/NTFS
/dev/sda4 30893 60802 240245761 5 Extended
/dev/sda5 30893 59584 230467584 83 Linux
/dev/sda6 59585 60802 9777152 82 Linux swap / Solaris
On this case is on '/dev/sda5' so we have to mount it:
(replace 'sda5' with the partition name in your case)
sudo mount /dev/sda5 /mnt
5. And then install grub:
6. Reboot and verify that all is working fine.
sudo grub-install --root-directory=/mnt /dev/sda
Hope that helps.