Two methods; I know both will work.
Grub gets borked enough, so I'm posting a couple links - by Xaos52 and hackerdefo
Reinstall grub2 - chroot into installation (http://crunchbang.org/forums/viewtopic.php?id=15351)
How to recover/reinstall grub2 (http://hakerdefo.blogspot.in/2013/01/how-to-recover-deleted-or-corrupted.html)