A Microsoft Advertising Help thread shows that a small little thing like using disallowed characters can put a complete halt to your other ads in the adCenter network.
I am not sure what this specific advertiser used as disallowed characters, I assume they used some funky HTML entities, like a ♥ or a ♠ or maybe funky arrows. But I had no clue how serious of a penalty it is. Where it would disallow the advertiser from advertising other sites or using other ads without the disallowed characters.
Ricky Poole from Microsoft said the advertiser received the "maximum QBR penalty," (QBR = Quality Based Ranking). But after careful review of the account, Microsoft manually removed those disallowed characters and the account is now active again.
We have investigated your account and found you had a maximum QBR penalty for disallowed characters in your ad text. However, we have reviewed the account and they have removed these special characters. Thus, we have removed the penalty. As the QBR penalty is no longer on the account, the quality scores will most likely start to increase, and hopefully the impression volume will as well.
Hope this helps! Please let us know if you should need any help going forward.
Forum discussion at Microsoft Advertising Help.