Unfortunately, that data was only in your local browser memory.
You had the window open for too long and the browser session timed out, causing you to lose any unsaved data.
When you login, if you tick the "remember me" box (or whatever its called), you won't get logged out with long browsing sessions -- however, you may still be at risk for losing unsaved data if the session is really long.
This is a feature of your browser, and has nothing to do with WikiTree.