It looks as though the error is coming from the AB Pagination add-on. Have you updated it recently?
As to why it's suddenly appearing, it's possible that the error may have always been there and it just wasn't shown. Maybe the server it's hosted on has had it's PHP
error_reporting settings changed in it's INI file to allow PHP Notices to be visible?
This blog post tells you how you can turn errors and notices on/off in code. So you would put the code it suggests in the
However, I'd suggest getting to the root of why the error is being caused in the first place, rather than putting a sticking plaster over it. I notice you've asked the question over at Addon Bakery, so hopefully the developer will give you a satisfactory answer.