i purchased acrobat pro a few years ago and most recently upon opening the app i got a "not authorized" on my display. what's that about?

Adobe have told us the following. That sometimes people sell genuine product that they are not allowed to sell. For example, they buy a business license for 100 users, but then sell them as separate products. Also sometimes there are made up serial numbers by some secret method. They use all sorts of tricks. Eventually, Adobe find out that the licenses have been sold in unauthorized ways. What can they do? Of course, they block this license for which they were not paid. This leaves the purchaser, who was scammed, in a difficult place. But it's unfortunately the case that if you buy something stolen, even after many years, you can lose it without a refund.

If you purchased from a reputable supplier, contact them about it. If you purchased from eBay, you can expect this sort of thing.