You have Windows and Linux on the same hard disk and you choose GRUB as boot loader. At boot time you see a menu to choose Windows or Linux. But one day you reinstall Windows and it rewrites your MBR and the menu has gone, now you only can boot Windows.
So how do you restore GRUB after Windows overwrites your MBR/boot loader? Easy ... !!!
- Boot with Linux Live CD/DVD.
- Login as root or guest.
- Click menu Start > System > Configuration > Boot and Init.
- Run Redo MBR and then restart.