1st option : get a CD including Boot-Repair

The easiest way to use Boot-Repair is to burn one of the following disks and boot on it.

Boot-Repair is included in Ubuntu-Secure-Remix (multi-languages, ok for Wifi, LVM and RAID, 32 or 64bit, based on Ubuntu 12.10, Boot-Repair shortcut in the desktop, the 64bit version is UEFI-compatible)
Boot-Repair-Disk is a CD starting Boot-Repair automatically. Both 32&64bit and only 360Mo, but: English only, no Wifi, no LVM, no RAID.

Remark : you can also install the ISO on a live-USB (eg via UnetBootin or LiliUSB or Universal USB Installer).

2nd option : install Boot-Repair in Ubuntu

- choose "Try Ubuntu"

- connect internet

- open a new Terminal, then type:

sudo add-apt-repository ppa:yannubuntu/boot-repair && sudo apt-get update

- Press Enter.

- Then type:

sudo apt-get install -y boot-repair && (sudo boot-repair &)

- Press Enter

launch Boot-Repair from either :
- the Dash (the Ubuntu logo at the top-left of the screen)
- or System->Administration->Boot-Repair menu (Ubuntu 10.04 only)
- or by typing 'boot-repair' in a terminal
Then click the "Recommended repair" button. When repair is finished, note the URL (paste.ubuntu.com/XXXXX) that appeared on a paper, then reboot and check if you recovered access to your OSs.
If the repair did not succeed, indicate the URL to people who help you by email or forum.