Here are three different reset options for the iPhone you can try. If they don't work you best option is contacting iPhone Support.

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1) If an application (such as Safari) is frozen, just press and hold the iPhone’s home button for about 5 seconds, or until the home screen shows up. (This didn’t work for my iPhone so we had to perform a soft reset).

2) To perform a soft reset, press and hold the iPhone’s Sleep/Wake
button and the Home button for several seconds, or until you see the
apple logo on the screen. (This action did reset my iPhone and I was
very happy at this point). The guys that helped me said that
continually performing soft resets can cause problems however.

3) Hard resets are not recommended, but are required when a soft
reset won’t do the trick. The Apple Geniuses wouldn’t tell me the key
combo to perform a hard reset because it restores the iPhone to a state
that’s unusable. Users risk losing data and you must synch your iPhone
with iTunes to make it function after a hard reset.

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Source(s):
http://www.notebooks.com/2007/07/02/how-to-reset-an-iphone/

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