When starting Lightroom, hold down ALT+SHIFT on Windows or OPT+SHIFT on Mac. Lightroom will start completely reset to default after you confirm that you want to reset preferences.

How do I start over in Lightroom?

Lightroom Guru

Or if you really want to “start over”, simply do File>New Catalog from inside Lightroom, and create the new catalog in the location of your choosing.

How do I reset my Lightroom photos?

With the photos selected, press Shift-Cmd-R or Shift-Ctrl-R to Reset the photos’ Develop settings. (In the Library module, the Reset command is under the Photo > Develop Settings menu.) Be careful when resetting; it will remove any adjustments you’ve made to the selected photos.