Since you have not mentioned which service, I'm assuming this is the google adwords display network. You can go to your campaign -> Display Network -> Placements. Here you can see which sites have shown your ads which were clicked. You can see if you impressions have dropped off, or whether the CTR has gone down.
Potentials reason:
1. Advertiser(s) increased their bids, thereby locking you out of the network. You may have to increase your max CPC to get your ads shown again.
2. Your quality score has gone down