Welcome to the forum.
Sounds like you've reached your device limit. If you've watched on four different devices in the past then the device limit rules kick in, where you can now only add one new device a month, even if you delete several.
Best thing to do is to have a word with Live Chat, explain what's happened and see if they can reset your devices for you. To get there go to the following help page
and click the green Contact Us button near the bottom left of it (not the How to Contact Us link right at the bottom as that takes you somewhere else) then choose the Live Chat option on the following page.
NOW are looking for users to join their Customer Feedback Group and help improve the service. If you're interested read more here and sign up.