Same with my blog. It appears on search, but one day they decided something fishy was going on with the ads and simply terminated the service. The automated e-mail said I could appeal and the appeal resulted in a "yes, it pretty much was something fishy" automated message. No further information, just silence and a warning that they would ignore messages about this subject from that point on.
Which has a list of different things like adsense you could use. Out of curiousity, is there any reason why you can't switch to using these companies for advertisement? I can see how google has a complete monopoly on ad-words, but surely adsense is a free market.
AdSense needs competition. Badly.