iPhone 3G - Uninstall the jailbreak from within the phone
Put your iPhone in DFU mode and that should help you restart your iPhone once again. 

To put your phone in DFU mode, do the following:


Connect your iPhone to a computer with iTunes. 
Press and hold the lock button for 10s. 
Without releasing the lock button, press and hold the home button for 10s. 
Release the lock button but keeping holding the home button till the iTunes logo comes on the screen.
Your device is now in DFU mode. iTunes should automatically detect the device in recovery and prompt you to restore it. 
Follow the steps and you should have your iPhone working again.