LUCKILY! You can get it back! Emacs' undo history provides a very nice system of both undo and redo.

Try typing some text, or doing some various complex things, then type undo a few types. Now move the cursor, or do something that "interrupts" a sequence of undos. Now start typing undos again. You're now reversing your undos!

The emacs manual includes more information on undo/redo... it's a pretty nice system. Of course, like anything in emacs, more can be done.