Sometimes, and it’s rare, if you can’t undo (command-z produces a warning signal), it means something went wrong.
If it isn’t saving, duplicate the Library on the Finder level, or export an XML of the entire Library.
I don’t know what happened in your situation, though. It seems very odd that FCPX wasn’t saving at all. That means that some sort of read/write error might have happened.
Do you store your libraries on the boot drive?
Jeremy