Sarah's Scribblings, that her Adsense had disappeared on 12/21/2011 and when she clicked on her Monetize tab it asked her to sign up for Adsense. So she went to my Simply CVS blog and sure enough mine were gone, as were my blogging buddy's Frugality Is Free blog. We thought there was a glitch and it would be fixed by morning, but it wasn't.
So I went searching and found nothing. I did find a reference in an older post about a code change and wondered if that could be it, but why would it happen now? When I looked at my "design" page the Adsense widgets still showed up. I clicked on one and it showed my
correct account number but I did notice it showed it was a banner ad and I had chosen it to be a links ad. I checked another links ad and it, too, now showed chosen as a banner ad. Hmmmm. You can only have three banner and three link ads--if these were recording as being banner ads then I had too many on.
I clicked to edit each ad widget on the design page and removed it. Then I began adding them back in one by one. It worked! Somehow the codes in the widgets got mixed up and caused them to be taken off because there were too many of the same type of ad. This was true of the other blogs, too.
So if your Adsense ads disappeared and you are sure they haven't canceled your account, try my quick fix and see if it works.