Im sorry to hear and I dont have any real solution (I just bought Maya, cant stand with the watermark in PLE), did you delete the history for the object when you were finished with it?
And it also good to often save to diffrent files, like scene01, scene02 and so on. Then its easy to go back. I do that before every major change, and also sometimes between. If somethings goes wrong, instead of trying to make it right (just undo or delete the changes doesnt always work, sometimes some things you can't see is still there, like parameter changes and so on) I just open up a previous version of the scene and start it all over from there until it works (and I have to do that alot because im a complete newbie).
Pardon for my bad english.