Boot Repair is the most commonly used tool to fix errors regarding Grub boot loader. First of all arrange a Live CD/DVD/ USB for Ubuntu and boot your system using this media (If you are looking to repair Grub, chances are you are already on the stage where system is un-bootable into current operating system, so yes its a valid request to use Live CD/DVD/USB at this point ).
Open terminal and insert command line...
|$ sudo add-apt-repository ppa:yannubuntu/boot-repair
$ sudo apt-get update
$ sudo apt-get install -y boot-repair