Boot from the install media you used, DVD or Flashdrive whatever you used.
While you are in the live session look for the Boot Repair Tool in a google search follow the steps to install it. Or copy then from here (Line by line pasted them on the terminal)
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install-y boot-repair
Once it is installed run it by typing
and that should take care of the problem
Let us know how it works for you