Do you mean you downgraded the OS on your computer?  If so, then you probably can’t run QB at all (??) and probably the fix is to reinstall QB.  Reinstall is just like install (just run the installer again.)

Resolution for '16 license code issue: 'went back to windows 7 pro from windows 10. lost quick books 16 accounting . said lost license code. how do I put license code in or reinstall' available: Yes.

The 16 license code issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the 16 license code issue yourself by looking for a resolution described in the article. If it is a complex 16 license code issue or you are unable to solve the issue, you may contact us by clicking here or by using other 16 license code support options.

For more information about resolving issues related to 16 license code in Quickbooks/QBO or to get support for the issue 'went back to windows 7 pro from windows 10. lost quick books 16 accounting . said lost license code. how do I put license code in or reinstall', click the link below: