1) Boot off the LiveCD
2) Open a Terminal and type in the following commands, noting that the first command will put you into the grub "prompt", and the next 3 commands will be executed there. Also note that hd0,0 implies the first hard drive and the first partition on that drive, which is where you probably installed grub to during installation. If not, then adjust accordingly.
$ sudo grub
> find /boot/grub/stage1
> root <the output of above command>
> setup (hd0)
Reboot (removing the livecd), and your boot menu should be back.