There are many ways Grub can get broken: installing Windows, messing around with Grub configuration files or just a faulty update. Luckily, there are also quite a few ways to restore Grub. Such a tool is Rescatux, an really easy to use Live CD that fixes the Grub in seconds.
Rescatux is a tool to recover Grub / Grub 2 that comes with a GUI so it's less "scary" for those of you who do not like the command line and can restore Grub to the MBR or update the Grub configuration with just a few clicks. Further more, the latest Rescatux version (released a few days ago) comes with an option to force a filesystem check which can help recover Grub or Grub2 when Grub cannot read the filesystem because of inconsistencies. It can Restore Grub to the MBR for most Linux distributions, however the update Grub option only works with Debian-based Linux distributions (Ubuntu, Linux Mint and so on).
To use Rescatux, simply burn it onto a CD or create a bootable USB stick, then boot the Live CD/USB and the Grub recovery window will automatically pop-up:
Then chose "grub-install" to restore the Grub and finally run "update-grub", then restart your computer:
If for some reason Rescatux doesn't work, you can try to do this manually: How To Recover GRUB2 [Linux]
Rescatux is supposed to work with both Grub (legacy) and Grub2, however please note that I've only tested it with Grub2 (it worked just fine for me - I broke the Grub for a virtual machine and then used Rescatux to fix it).
Download Rescatux (a single ISO for both 32bit and 64bit - 595 mb)
See also: Grub Customizer (can also fix Grub2)
[via Tux Planet.fr]