Have you tried to force the phone to reboot by holding down the power and home button at the same time? If this doesnt work, try this:
WARNING: THIS WILL RESTORE YOUR PHONE TO FACTORY SETTINGS!
1. Connect your iPhone to your computer and lanch the latest version of iTunes.
2. Press home+power button until screen goes black. When the screen goes black, keep pressing the home button but let go of the power button.
This should bring the itunes and usb thing back, but also display "iTunes has found an iPhone in recovery mode." on the computer. In iTunes, you are now able to reset the phone. Once you have reset the phone, try to launch it again. If it doesn't launch, hold down power and home at the same time until the phone reboots.
Apple support article: http://support.apple.com/kb/HT1808?viewlocale=en_US&locale=en_US
Hope this helped!
Message was edited by: lunkebo
Reason: Added link to Apple Support Article.