first step, is enter DFU mode, and run a restore. A restore is better than a reset, as it installes all new firmware, rather than utilizing the current firmware. should you have a corruption in the core files of your firmware, a reset will not fix it, while a restore will.
To enter DFU mode, follow these steps
1) Plug in the device.
2) Power down the iPhone
3) Hold the power button until you see the apple logo
4) without letting go of the power button, hold the home button for 10-15 seconds. (until the screen goes black)
5) Without letting go of the home button, release the power button, and continue holding the home button for 15-20 seconds.
If you have done it correctly, your iphones screen will be pure black, and your computer should be installing driver software.
From here, run the restore process, as instructed on itunes.