[webmasterworld.com...]
Check out AWA's post (msg. #6)
* Look under the ad itself, in the Ad Group view. Do you see the word 'disapproved' there? If so, check your email for an explanation of why. A disapproved ad will not be running.
* In the ad group view, do you see any keywords with one red asterisks? If so, your ad will not be showing for that keyword. If it happens to be your only keyword, then it essentially brings everything to a halt.
* Please check your campaign end date. You can do this in the campaign view, by clicking on the link that says 'Edit Campaign Settings'. End date is shown in item #3. Many new advertiser set end date for a one or two day test, then forget that they've done that.
* While in the page above, also check your recommended daily budget in item #2. This won't usually stop an ad, but it can certainly prevent it from showing all the time.
* While in that same page, check to see that your ad is still targeted to the correct countries/languages. It sometimes happens that advertiser change these settings by accident, and this can have major impact on impressions/clicks.
I hope one of these things turns out to be the issue, or that you've heard back from AdWords support.