iphone 3gs help!!!!!!!!?

im trying to update my iphone but after the loading bar on the iphone is a little past the beginning just freezes up and wont move and if i try to disconnect the cable or turn of the iphone it just says reconnect it to itunes then restore the iphone i did that and now half way it froze up again...what can i do to either get it back to normal or complete the update?
 
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