(1) You need to know the partition where Mint was installed. I used /dev/sda3 as an example but please adjust it for your case. If you are not sure ask question here!
(2) Boot up a Linux Mint Live CD & select a terminal.
(3) Obtain root privilege because you have to work with system-related commands by issuing this command
For Live CD no password is asked.
(4) You create a mounting point to mount the Linux partition which I assume to be /dev/sda3, mount it and restore Grub in the MBR of first boot disk /dev/sda
If Grub does not report an error do a reboot and Grub menu will appear.
mount /dev/sda3 /mnt/dude_said_mount_here
mount --bind /dev /mnt/dude_said_mount_here/dev
grub-install --root-directory=/mnt/dude_said_mount_here /dev/sda