A client of mine recently asked if I had switched his campaign off. He had searched for a few of his keywords and couldn’t see any of his Adwords adverts in the Sponsored Links results area. I often see comments on bulletin boards and other forums along similar lines, so I thought it was worth a mention here.
The reason why ads do not show is most likely to be because Google spreads the delivery of adverts throughout the day at a rate that will best achieve the campaigns daily budget. It doesn’t want to blow your entire daily budget within the first hour of the day (although it might do if your click cost is a high proportion of your daily budget!) and so it will stop delivering your adverts if the click rate gets ahead of itself.
As a campaign manager, I quite often manually pause a campaign for similar reasons. I regularly set campaign budgets high so that my adverts get plenty of exposure. When I introduce new advert groups (or adverts or keywords), they are often more successful than I had expected, generating many more clicks than I am targetting. So, short-term, I pause the campaign whilst I adjust it so that it delivers the right blend of quality and quantity for the price agreed with the client.