Shortly before Matlab expires (but, unfortunately, some time after it starts giving warnings) the annual campus Matlab license will be renewed. It will renew automatically if you launch after it is expired, but if you want to do it ahead of time you may do so by going to Update Current Licenses item under Help / Licensing. If asked to log in to Mathworks to refresh the license, use your JHED@jh.edu as the login ID.
Matlab is, frankly, a pain about licensing. If you’re still having problems you should check our page on installing Matlab from scratch for extra information.