The Google AdWords Blog wrote that yesterday at about 4:30pm to 7:30pm (EST), there was an AdWords ad serving bug that prevented ads from being displayed.
Google said the ads were not showing on Google.com and Search Partner sites at around 4:40pm. Google posted the issue at 6:30pm and updated the post about an hour later saying the issue has been fully resolved.
So it appears that the ad serving issue lasted for about 3 hours or so.
Google apologized and said:
Again, we apologize for any inconvenience this may have caused. Our engineers are working to ensure that this type of issue doesn’t occur again.