Did you try an "undo" after you did that? Maybe you didn't realize what had happened at the time.
If you haven't saved the file, of course your last-saved document should be intact.
OOo does make an auto-save file (in the backup folder as bobban already mentioned) every 15 minutes by default. However once you exit OOo, that will be removed.
I've used software that would actually stop making auto-save files when the size of the document decreased drastically like this, just to try and protect against losing work like this. It would warn you when that happened. I think OOo could use some enhancement in this area.
AOO 4 • Linux • Fedora 17