Try to force your phone into recovery mode. Disconnect it from the computer's USB cord. Turn it off if you can by holding the sleep/wake switch until you see the red slide to off. If you can't turn it off, please continue. Press and hold the home button while plugging it into the computer's USB cord. Continue holding the Home button while iPhone starts up. While starting up, you will see the Apple logo. When you see "Connect to iTunes" on the screen, you can release the Home button and iTunes will display the recovery mode message. See http://support.apple.com/kb/HT1808 for details.
Then you can restore your phone: http://support.apple.com/kb/HT1414