- boot to an Ubuntu LiveCD
- mount your linux / partition from the LiveCD environment
- chroot over to it
- install your system’s grub back to the MBR

http://www.howtogeek.com/howto/ubuntu/reinstall-ubuntu-grub-bootloader-after-windows-wipes-it-out/ is a howto that covers some of the basics, and chroot and mount aren’t really hard to operate, so you should have most of the tools you need from that howto.