YouTube has been intermittently testing strategies to show either ads or their anti-adblocking wall. However, in some cases we've found that your settings might have changed which is why ads are suddenly appearing. Please try the steps in this article: https://help.adblockplus.org/hc/en-us/articles/26653087560595-Ads-appearing-on-YouTube. If those steps don't help, please submit a support request and we'll be happy to assist you: https://help.adblockplus.org/hc/en-us/requests/new.