If it is a problem with Grub, I have fixed Mint installations with no working Grub many times.
Here is what I would do - boot my Rescatux/SuperGrub2 CD, SuperGrub2 boots my installed OS for me, I run "sudo update-grub" and "sudo grub-install /dev/sda". It is all quick and simple.
If you want to try the Rescatux/SuperGrub2 CD, here are some details:
Download Rescatux http://www.supergrubdisk.org
and burn it to a CD.
Boot the computer with the Rescatux CD. When Rescatux starts you will get a menu to start Rescatux or SuperGrub2. The SuperGrub2 option is the one you want. After chosing SuperGrub2, the next menu will give the options of booting any installed operating system on your computer ("Detect any Operating System") or any loop mountable iso files ("Detect loop bootable ISO's"). When you chose the "Detect any Operating System" menu item, the program will search your drives for your installed operating systems and create a menu for you that will allow you to boot any one of them. It will be a slow process to search your drives and create that menu, so be patient and let it complete.
Then after you boot Mint run the following commands in a terminal:
sudo grub-install /dev/sdX
- where X is the drive you want to install Grub to. (The drive Mint is on. sda? sdb?)
Tutorial and Troubleshooting with SuperGRUB2: