How to manually install boot loader?
Grub failure when installing 12.4 alongside Windows 7 (50 GB partition for 12.4), need to manually install bootloader. Where can I find this?
install Boot-Repair in Ubuntu
What you need to do is create a Boot-Repair Live USB or Live CD. This will repair Grub and allow you to boot Windows and Ubuntu again. You can find instructions on creating a Boot-Repair Live USB or CD here.