Here is THE Rescue Disk that you should keep close to your computer !
HOW TO GET AND USE THE DISK:
(1) DOWNLOAD BOOT-REPAIR-DISK,
(2) Then burn it on CD or put it on USB key via Unetbootin,
(3) Insert the Boot-Repair-Disk and reboot the PC,
(4) Choose your language,
(5) Connect internet if possible
(6) Click "Recommended repair"
(7) Reboot the pc --> solves the majority of bootsector/GRUB/MBR problems

GET HELP: by Email (boot.repair ATT gmail DOT com)
HELP THE PROJECT: Translate, or Donate (Paypal account:boot.repair@gmail.com)