sorry for my bad english! :) it is because when you install windows after linux(ubuntu) the windows loader takes places on the hard disk, to do: It consists of booting thanks to Super Grub2 Disk. Once in your system you will recover GRUB with a simple command. http://www.supergrubdisk.org/ download this file and then boot then in a usb or disk, then boot this from the boot options or bios then in the super grub2 select:
Detect any os
your Ubuntu (you have to select it from the list of detected OS)
Once you have started in your distribution you will open a console and load the Grub in the MBR by executing the following command:
Grub-install / dev / sda when sda is your hard disk!
if you want to read this in spanish and more detailed, http://www.guia-ubuntu.com/index.php/Recuperar_GRUB