Computer boo-boos happen all the time. Just this morning, an accidental graze of my laptop's touchpad inadvertently selected a big chunk of text in Word--which I then overwrote as I continued typing on autopilot.
Before I had even a moment to panic ("My words! My precious words!"), my fingers instinctively landed on Ctrl-Z, the keyboard shortcut for Undo. Presto: The paragraph returned as if by magic, and I continued on my merry typing way.
Although this is Intro to Basic Computing for Toddlers 101 to many users, I continue to meet people who don't even know Undo exists. (If you're one of them, yes, I just compared you to a toddler.) They make a mistake, say "Oh, shoot," and then start recreating whatever work they just lost or messed up. Madness!
In a nutshell, Undo undoes whatever action you just took, whether it was erasing a paragraph, deleting an e-mail, editing a snippet of video, or adding numbers to a spreadsheet cell.
Just about every application known to man has an Undo feature--usually in the Edit menu, and almost always a Ctrl-Z-press away. What's more, many programs support multiple levels of undo, meaning you can reverse not only the most recent action, but also the one before that, the one before that, and so on.
Need to undo an undo? Some apps offer a "redo" option: Tap Ctrl-Y to un-reverse whatever action you just reversed. Or, if you're more of a menu person, look for Redo in the Edit menu. It won't be far from Undo.