That you have an ad created for the search terms. I know that seems obvious but that's what I did a couple of year's ago.
Make sure you have set both your budget and max. per click high enough for the terms you have chosen.
That you have not accidentally clicked the pause campaign button.
Make sure that your countries and languages are correct in the campaign settings. Sometimes these get accidentally messed up because of scrolling.
Double check everything.
Write Goggle AdWords support again.
It's an excellent archive of threads, many offering solutions to common problems experienced by users.
Syzygy