put into DFU mode.
1)plug in your iphone
2)hold the power button for 2-3 seconds.
3)still holding the power button, hold the home button for 8 seconds.
4)let go of the power button, then still hold the home button for 8 more seconds.
5)reverse the steps and this may take a few minutes to reboot. if this doesn't work restart the steps.
(ps. he does not say step number 2 but i find it works better.)[http://www.youtube.com/watch?v=34v8HMC9B98&feature=related]