Why is Adobe Acrobat suddenly telling me my license is invalid after I've been using it for the last 3 years?

Exactly what version of Acrobat Pro did you purchase a license for in September 2016 and from whom did you purchase that license?

What is the exact wording of the message that you are receiving?

Note that by September 2016, the only versions of Acrobat that were being licensed by authorized Adobe resellers were Acrobat 2015 (perpetual license model) and Acrobat DC (subscription). If your version from September 2016 is Acrobat 10 or 11, for example, it is likely that you may have some questionable goods.

In any case, you may want to look at Invalid or revoked serial numbers, Adobe Genuine Software Integrity, and if necessary, contact Adobe at +1 800-833-6687. They should be able to help determine why the license is now being flagged as invalid and if this is in error, assist you.

Other than Adobe Customer Support, we really cannot assist you here with this type of issue.

Good luck.

          - Dov