If the undo is grayed out and you have NOT quit since you deleted the files. then you have been bit by the "Undo Killer" BUG... Basically at some point FCPx will quit recording your undos and in that process it will gray out the undo history.. Not very great but the unfortunate truth...
In your case...
Perhaps there is hope
1) Before quitting FCPx
2) In the Finder goto the Project
3) copy CurrentVersion.fcpproject to the i.e. desktop
4) copy the backups folder to the desktop folder as well.
5) Quit FCPx
6) in the orignal projects folder - move the CurrentVersion.fcpproject to the trash
7) in the orignal projects folder - COPY (Dont MOVE) the file ....fcpproject from the backup folder into the main project folder (Where the CurrentVersion.fcpproject used to be)
8) give it the name "CurrentVersion.fcpproject" without the quotes
Now boot FCPx.....
I was once able to save a project like this.... NOT saying you will. But at least it is worth a try.
If it does not work... Then can move back the files/folders you copied to their old locations... QUIT FCPx before doing this.