Symptoms
ArcGIS fails to license because your license code has already been used. This can happen after reinstalling your operating system for any reason (virus, performance, etc), or after reinstalling ArcGIS, or upgrading ArcGIS without first deauthorizing licenses.
Resolution
Licenses can be reinstalled, but if you didn’t deauthorize before uninstalling ArcGIS, you’ll need to deauthorize your existing license still. See the Esri article here for instructions on resolving the issue. If you reinstalled the operating system on the computer that originally had ArcGIS, then you will need to contact Esri support to resolve this issue.