This could be caused by a number of scenarios. One of the most common things to check is whether you’re actually connected to the internet (preferably WiFi)? I have seen many people not realise they weren’t connected, especially after an iOS update.
In addition, two of the most common things to try are:
- Doing a full reboot of your iPhone (i.e. holding down the Power and Home buttons simulatenousy for about 10 secs until you see the Apple logo on a black screen)
- Logging out and back in. (See steps below)
You don’t actually clarify what version of iOS you’re running, but here are some generic steps to take:
- Go to Settings >iTunes & App Store
- Tap on your Apple ID at the very top
- Tap on Sign Out
- Now sign back in
Hope this helps. Let us know you go, as there are other possible ways to fix this.