Just heard that Yahoo PPC search marketing is down for some advertisers. So don't panic if yours is not working. Here is what I heard from Yahoo.
"We are currently experiencing a technical issue that is causing the ads of some advertisers to not be displayed properly to users. This may cause your account to receive a lower volume of clicks than normal. We are aware of this issue, and are working diligently to resolve it as soon as possible. We apologize for any inconvenience this may cause you."