The easiest way to get around this is to insert a bootable disk or usb like you are going to do a fresh install. You'll want to start up to the live desktop, open a terminal, and enter the following:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install boot-repair
Once it is all done installing, it will launch and you should be able to get it to work with just the recommended repair. If that doesn't want to work for you when you reboot, follow the same steps and then go into the advanced options and you can select the options to reinstall grub, select what OS to boot from, where to place the reinstall of grub, etc.
Note: you have to do this through the ubuntu live desktop from a bootable disk or usb. Do, not reinstall ubuntu or anything like that when you boot into the live session. Just enter that code into the terminal and you should be all good.