What did it say, specifically, when it rejected it?
It may have been blacklisted, which could be the case if you were originally scammed into buying counterfeit software or if you have shared your serial number with a number of others.
Or you may just be having a technical problem reaching Adobe's servers because something in your system changed making it impossible for the software to reach Adobe's servers.
The devil is in the details here.
You may ultimately need to call Adobe Customer Support, though... There's really nothing we fellow users can do to manipulate Adobe's activation database.
The serial number you entered is not valid. Click OK to retry entering the serial number
Sent from my Verizon Wireless 4G LTE smartphone
Adobe has published a Licensing Repair Tool that may help:
If that doesn't work, it's possible that a complete removal and reinstall might help.
Otherwise you're going to have to talk to Adobe on the phone for activation issues.