Saw an article today. Seems it's been an issue today, for many users, worldwide. The suggested fix it to log out then back into your account. You do this using the Apple ID button found at the bottom of most tabs in the the App Store.
Same thing for the other iStores (iTunes, iBooks, etc.). Each store maintains it's own log in informations, so you'll need to do it for all stores.
Most likely it will start working on it's own in a bit, as it appears to have done for SciFan.