Don't know what's wrong. All of a sudden my ad isn't being placed at all. I didn't change any of my keywords or my bid, and they've suddenly stopped appearing altogether. Adwords tells me that my ad is active for all my keywords, so why isn't it there? I usually have quite a good position.
Check to make sure nothing is eating up your budget quicker than you want. If your budget for the day is used up the ads will not appear. Also check to see if the keyword has been registered as a trademark of a company. Adwords makes efforts to try stop competitors biding on another companies registered name or trademark. That's two possibilities. Hope it helps narrow it down if indeed this is not the cause.
There's no way for us to know. Check this- https://adwords.google.com/support/bin/answer.py?answer=6105