Did you receive an email from HubPages about this? Does the hub in question have an automated or moderated warning at the top when you go into edit mode? You can also see those warnings at the top of your hub without going into edit mode. If there's a warning, that warning should tell you what might be wrong with your hub.
If you think there might have been a mistake about ads being disabled in your hub, maybe you can ask HubPages about it. You could ask for clarification on why the ads in your hub have been disabled. You can contact HubPages by using the help wizard. Here's the link for that: http://hubpages.com/help-wizard/ . Hope this helped.