I remember reading about this somewhere, but for the life of me I could not find the link for it again (I did find a reference to it here). So once again for my own sanity (and saving me a few minutes of fumbling around): here is how you can recover from changes made to your templates, when your undo history doesn’t quite stretch far enough back.

• In your navigator view, right click on the file in question.
• From the context menu select Compare with > Local history
• The pop up that you are now presented with now displays you current state in the bottom left pane and in the right hand bottom pane you can see a historical view of the file in question.
• The top pane displays the known history of the file, so simply step through the history until you find that line of elusive code you had accidentally “lost”

You can also increase the number of days (defaults to 7 [or 50 entries per file]) to keep in history from Window > Preferences > General > Workspace > Local History.