Sometimes for some reason when you installed Microsoft Windows or other operating systems, your GRUB installation will be overwritten. This will cause your Ubuntu operating system unable to boot.
Here’s how to solve the problem using Ubuntu LTS Live CD:
1. First boot the Ubuntu Live CD from your computer
2. Then, open terminal-emulator application, and add boot-repair application to Ubuntu repository
[bash]
$ sudo add-apt-repository ppa:yannubuntu/boot-repair
[/bash]
3. Update repository and install boot-repair
[bash]
$ sudo apt-get update
$ sudo apt-get install -y boot-repair
[/bash]
4. Then run boot repair by typing “boot-repair” at the bash prompt
To begin repairing/reinstalling GRUB, click on “Recommended Repair” at the application screen:
The “Advance” button offers other option that lets you customize the boot option.
Boot-repair lets you to:
1. Reinstall GRUB bootloader
2. Change the default OS to boot
3. Place boot flag on any partition or devices.
4. Add extra kernel option
5. Restore MBR (only when necessary!)
6. Repair filesystem (only when necessary!)
Official Boot-repair website: http://sourceforge.net/p/boot-repair/home/Home/
Ubuntu Community Page on Boot-repair: https://help.ubuntu.com/community/Boot-Repair