Question: Why has my AutoCAD suddenly become unlicensed?
Answer: If your AutoCAD was previously working then the next day it becomes unlicensed you may have upgraded to a collection and/or subscription license. Your Autodesk contract manager would need to logon to their Autodesk portal to assign a subscription license to your Autodesk user account. Then, providing the product key is the same (e.g. 001J1 for AutoCAD 2018) on your installation you could open the product and enter your Autodesk logon details to verify the license.