How long has this been going on? Do you see any notifications with an exclamation point icon? It could be related to the new Google Play Services update being pushed to your device--but that should only cause a very temporary error.
Otherwise, have you tried going to Settings>Apps>All, selecting Google Play Store, and Force Stopping, then Clearing Cache/Data? The other thing to do would be to go to Google Wallet and make sure your credit card is valid--maybe the one on file expired.