I don't think you need to reinstall. Doesn't ubuntu include a live cd? Boot that one (or in fact any other live linux cd), chroot to your ubuntu root, mount boot and execute grub-install.
Of course first verify that it's not only a booting sequence gone awry :) In details: 1) Boot live cd. 2) Get to a console as root. In ubuntu: Applications->Tools(?)->Terminal, sudo gnome-terminal. 3) Chroot to root: i) mount your ubuntu root somewhere (let's assume /media/ubu) ii) chroot /media/ubu 4) mount -a 5) grub-install /dev/sda1, where /dev/sda1 is your booter. Good luck, Klaus
