AdSense on one of my sites has disappeared. The ad unit shave been replaed with public service ads and hte link units have been replaced with 'view ads about' search boxes.

Has this happened to anybody else? How can I fix this?

Have you checked out your google account. The time it happened to me was because they did not like the site I had them on.

You probably submitted your sites to Google search engine a few times or more a month they probably ban your site. My friend did that and could not get adsense ads on his site back but he could get another site that could have adsense ads unless he submitted it a few times a month. do site:yoursite if google really ban your sites.

Google also does that when they can't find content on your site. I had this issue when designing a imagehost site. Changed anything around recently?

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"I find your argument strewn with gaping defects in logic."

nothing to worry as long as you can still login to your adsense account. it happens to me as well if gugle can't fing the most approriate ads for your site i guess...