Oh *** pepe2907, I didn't even realize he said on the same partition lol. abbeyengr, pepe is right. You can't install an OS on the same partition as another. That's basically installing an OS over an OS overwriting the OS that already existed. When you do that, you're overwriting partition tables, master file tables, boot loaders, everything. The previous OS no longer exists. However, if you said you have an important file on another partition, then slave your hard drive into another computer. Try a Windows computer and if it doesn't work, then slave the drive into another linux computer. You should be able to read the other partition.
In the future, make sure you always install OS's on their own seperate partition.