Has your grub has been crashed. Here is a way out to reinstall the grub into your machine !
Boot your machine with the ubuntu installation media. And click on the TRY UBUNTU.
Open the terminal and follow the commands:
#sudo add-apt-repository ppa:yannubuntu/boot-repair
#sudo apt-get update
#sudo apt-get install –y boot-repair
Click the required configuration of the grub and click next………..
And its Done