Use Boot Repair to fix your bootloader.
1 - Live CD
First, you'll need to boot from a Ubuntu Live CD.
2 - Repair the bootloader
To restore the Grub bootloader, follow these steps.
Open a terminal from the Live CD, and run the following commands:
sudo add-apt-repository ppa:yannubuntu/boot-repair
sudo apt-get update
sudo apt-get install -y boot-repair
boot-repair
After a few seconds, Boot Repair's main window will open.
Click on Recommended repair
.
You're done!
Reboot to test everything out.
If everything went well, you should be up and running now, with both Windows and Ubuntu appearing in the Grub menu.