Anyone who's had an iPhone or iPod touch for a while knows that sometimes apps just stop working. In some cases a single app will stop working while the rest seem just fine. The first step is reboot your phone, but sometimes that doesn't work. In that case, the solution is generally to delete the app from your iPhone and the re-download it.
However, in some rare instances, all of your third-party iPhone apps can stop working, even after a reboot. I'm not 100% sure why this happens, but it seems to be an issue with the FairPlay DRM database becoming corrupted. I've found that the best solution is to download another app from the App Store.