Install bootable Ubuntu 14.04/15.04 (may be applicable to other versions too) in your pendrive. Try Ubuntu, open terminal and type the following commands.
1. sudo add-apt-repository ppa:yannubuntu/boot-repair
2. sudo apt-get update
3. sudo apt-get install boot-repair
4. boot-repair
Finally a window will come up showing that the repair has been completed.
