Windows 7 was pre-installed on my machine. Ubuntu 11.10 doesn't boot after I installed it. Once Ubuntu finished its installation, I restarted.
After that, it just boots straight to Windows 7, no sign on Ubuntu at all.
If there was no error while installing then you can re-install your GRUB as follows:
sudo fdisk -lto see all your partitions and exact name of your linux one (sdxy) where
xmeans the hard number (a,b,c,...) and
ythe number of partition (1,2,3,4,...)
sudo mkdir /media/root
sudo mount /dev/sdxy /media/root
sudo grub-install --root-directory=/media/root /dev/sdx(without y number!)
Something likely failed with the Grub installation.
There are multiple ways to tackle this.. but ill give you the short one
sudo grub-install /dev/DRIVE
DRIVE is the drive first starts as selected in the BIOS.. usually this is /dev/sda