The ad has failed to load (that is why it wanted to call the missing didFailToReceiveAdWithError: delegate method). According to the docs, you are supposed to hide the view when you receive this error, so that the user doesn’t have to stare at the ugly blank spot.
You can see what is the reason for the failed ad load if you implement the didFailToReceiveAdWithError: method and look at the error passed there. The most common cause is not enough inventory – it happens even with the test ads (to emulate real life I guess).