This is a common problem with most new Ubuntu(or any Linux) enthusiasts. You install Ubuntu with a dual boot with Windows. Due to some freaking reason, you have to perform a Windows reinstall and what the !!!! Now you can't see Ubuntu in the OS's choice while booting your computer, kore precisely you don't get an OS choice menu.
Ok, here's a simple straight forward trick to regain your lost GRUB bootloader.
1. Boot your computer using an Ubuntu Live CD.
2. Press Alt+F2, type xterm
and hit enter.
3. Now enter following commands:
4. The output of previous commands will be like (hd0,0) or (hd1,2) depending on where you had installed your Ubuntu.