My iPhone is not able to download an app

Since two days my Iphone is not able to download the Facebook-App. Also I tried to download another app. The download started. But before the download finishes a window opens: The app can not download at the moment. Also I tried to download another app and another.. The same result. Then I tried it on iTunes on my computer. Nothing, I install the app but the app didn't install complete. What should I do??