I had installed a Ubuntu 12.04 and Fedora 17 dual boot on my system. During the installation I had installed Ubuntu first and Fedora later. Fedora had recognized Ubuntu and added it to the GRUB OS selection list. Afterwards I installed some routine updates on my Ubuntu and after that I am just not able to see the GRUB OS selection anymore when I boot. I am unable to understand what happened, both Fedora and Ubuntu use GRUB 2.0. Also it seems Ubuntu is not able to recognize other existing linux operating systems; because in the beginning I had installed Fedora first and Ubuntu later, but Ubuntu did not recognize Fedora at all, while Fedora recognized Ubuntu when I installed the other way round.
Try installing the Grub Customizer with
When you open the app, it shows all the detected OS's (as well as old kernel versions). You can tick/untick the boxes to show/hide options at bootup.
It may be possible that Fedora is hidden. If so, you can easily fix it with the customizer.