This is why it is advisable always to regularly save your map editing work (or scenario editing for that matter) just like any other computer related business like working on a Word document or programme. Since then you have a known good backup to return to if it all goes pear-shaped on you!
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The Extended map editor for CD owners as I recall has an undo function - and also it auto-saves on a timed basis as well IIRC. (Been some time since I worked with it, maps are not my thing)
Andy