Try resetting it by holding the power and home buttons at the same time until you see the Apple logo, then releasing. If it won't reset after 30 seconds or so see if you can restore it in iTunes by connecting your phone, going to the Summary tab of your iTunes sync settings and clicking Restore. If you can't, you'll have to force it into recovery mode to restore it, as explained here: http://support.apple.com/kb/ht1808.
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