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
After a few seconds, Boot Repair's main window will open.
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.