Forum Moderators: martinibuster
Also, did you perhaps specify an alternate ad color? This could seem to show a blank space, instead of running an alternate ad or PSA if there were no targeted ads to display:
Alternate Ad Color
A new feature has been added to the alternate ad selection in your ad layout code: the solid color picker. Simply select a color code from the color picker on your Ad layout code page, and AdSense will fill your ad unit with the selected color instead of PSAs when targeted ads cannot be served.
If it isn't those kind of issues, there are a few scenarios. AdSense will sometimes disappear from a site for short periods of time, and it often seems to be when the AdSense team is checking over a site for some reason.
The second scenario is if the entire domain has been blocked for some reason. For example, if a domain has been suspended or rejected for AdSense, a block is placed on the domain so AdSense ads will not show up, just a blank space will.