If you haven't closed Lightroom, then just press Ctrl/Cmd-Z (or use the undo menu).
If you have closed Lightroom, then you have to:
* revert to a backup catalog, and/or
* read xmp (only viable if you saved xmp manually after editing, but before the screw up), or
* Download and run ScrewAutoSync plugin.
Thanks Rob, (I actually aborted the process when I realized it and also pressed cmd-Z, but apparently that did not go into effect...)
If I would revert to a backup catalog, can I choose how far I would like to go back in time?
Hi Jonas - you're welcome.
apparently that did not go into effect
Keep pressing it until it does - watch the bezel (black ribbon that displays info) - if you go too far, then press Cmd-Y.
If that doesn't do it (it should, if you haven't exited Lightroom), then you can revert to any backup catalog that you made, or had Lightroom make. You don't choose how far back you go in time, so much as you choose a backup catalog, which will be exactly as your catalog was when you made that backup.