The first thing to check is that you haven't reached your Daily Budget.
Click the little wrench icon by your campaign and check the amount set.
Then check the 'Total' you have spent today and see if they match, if they do, you will need to either increase your Daily Budget or wait until tomorrow for more traffic.
Please also check if the 'Status' of your campaign is 'Paused', to resume the traffic just click on the 'Paused' link and the status will immediately change to 'Active'.
Another might be that your website has been 'Declined' or 'Pending'. Please check the 'Status' of your campaign in the top right corner.
If none of these apply to you, please contact us.