I found this little gem about troubleshooting issues with the iPhone in our iPhone Troubleshooting Assistant, found here http://www.apple.com/support/iphone/assistant/phone/#section_1. Sections 1, 3, and 5 will resolve the issue if it is software causing it. Here is section 1 to get you started:
To restart iPhone, first turn iPhone off by pressing and holding the Sleep/Wake button until a red slider appears. Slide your finger across the slider and iPhone will turn off after a few moments.
Next, turn iPhone on by pressing and holding the Sleep/Wake button until the Apple logo appears.
Is iPhone not responding? To reset iPhone, press and hold the Sleep/Wake button and the Home button at the same time for at least 10 seconds, until the Apple logo appears.
If your device does not turn on or displays a red battery icon, try recharging next.
All the very best,