dual boot - Where did Ubuntu go?

I just formatted my PC and added Windows 7, alongside Ubuntu. I installed them in a separated disk partition so Ubuntu and my files should be ok, though I cannot find them anywhere. When I try to boot manually, my only choice is windows 7, same also on startup settings in control panel.
Ubuntu must be somewhere though. The disk has the half the size it's supposed too. But I can't access their partition. I didn't retrieve my files, and that's definitely a problem.
Can you help me please?