Do you have grub coming up at boot?
Grub Rescue - reinstall grub.
Boot and run from Live DVD
open a terminal, type these commands
1. sudo os-prober
see a list of OSes first.
here is a possible output:
mint@mint ~ $ sudo os-prober
/dev/sda1:Windows 7 (loader):Windows:chain
/dev/sdb1:Linux Mint 13 Maya (13):LinuxMint:linux
/dev/sdd1:Linux Mint Debian Edition (1):LinuxMint1:linux
Obviously yours may differ.
2.
sudo mount /dev/sdb1 /mnt ( mint root)
3.
sudo grub-install --root-directory=/mnt /dev/sda (main disk)
I would suggest you do a complete backup of your home directory.
Also see
http://www.cyberciti.biz/faq/linux-find ... ate-fstab/