AWA advises a member to check the campaign end date as a reason that ads suddenly stopped showing.
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Andystowell, one other thing to check: Your campaign 'end date'. You can check this in your 'Edit Campaign Settings' page.
Many advertisers who are just starting out with AdWords want to run a 'limited test'. So they set their campaign end date to a particular date in the very near future - say a week for example - and then forget about it.
A week later, everything comes to a screeching halt, and temporary mystification ensues.
I hope your particular case turns out to be something as simple as this. ;)