It is relatively easy to reinstall grub in a situation like you describe. Lets use an example.
sda3 Linux Mint complete install
If you are locked out of your Mint install you can use something like Supergrub to boot the Linux install. Supergrub is handy to have around as it can boot almost anything. Don't be tempted to try to fix things with Supergrub unless you know what you are doing. Supergrub is a very useful and powerful tool.
Once you have booted into your Mint desktop open a terminal and type:
The first line opens a grub terminal. The second line tells grub where the stage 1.5 and menu.lst file are located, (1st hard drive - 3rd partition). The third line tells grub where to install the grub kernel, (1st hard drive mbr). The forth line exits the grub terminal.
You have now reinstalled grub.