Is it a public service type of ad? Do you also see these ads after using a different browser or IP address and/or clearing your cookies?
Chances are that they either do not have any ads related to your content at the moment (usually because of tons of pageviews), or that your account is getting audited.
Also, you may want to do a "View Source" on your websites and see if there is any evidence of a XSS or SQL injection attack.