Which software is installed on your iPhone? The most recent version would be iOS 8.1.2. If your device isn't up to date, update it and see if this solves your network problem.
If it's up to date, reset it. This won't remove any data or settings, it just might remove a few glitches. To reset it, press and hold Home and Power button until you see the Apple logo. Let go of both as soon as it's visible. The iPhone will power up on it's own.
If this doesn't help, reset your network settings in Settings - General - Reset. This will remove all the information you set up for your network, also the Wi-Fi password. You'll have to set up the network again afterwards.
Hope that helps.