First of all, reset it: press Home and Power button simultaneously until you see the Apple logo. Let go of both. It will power up on it's own.
This kind of reset won't remove any data or settings.
A restart might also help:
Turn it off and on again.
If this doesn't fix your problem, go to Settings - General - Reset - Reset all Settings. This will clear every information you added (Wi-Fi passwords, wallpapers, ...). After resetting this way, you'll have to go through the Settings app and get everything back to the way you need it.
Your last option would be to use iTunes (computer) to restore it. Make sure iTunes is up to date, backup your iPhone and restore it.
Hope that helps.