I recently changed my contract and devices. When unplugging the cable box to send back, we cannot get the tv app to work, so need to reactivate the cable box. We've tried all the suggestions , but I think it needs to be added onto my equipment before it can work. Please help!

I recently changed plans and it did not include cable box but the app. The app won't work and now neither will the cable box. How can I get the cable box reactivated.