If you already own a Classic license and need to reactivate it:

: You can use the SketchUp License Manager by entering your serial number and the email address used during purchase. Activation and Use Constraints

Sketchup Pro Classic License Key New!

If you already own a Classic license and need to reactivate it:

: You can use the SketchUp License Manager by entering your serial number and the email address used during purchase. Activation and Use Constraints