Wirelessly posted (iPhone: Mozilla/5.0 (iPhone; U; CPU iPhone OS 2_1 like Mac OS X; en-us) AppleWebKit/525.18.1 (KHTML, like Gecko) Version/3.1.1 Mobile/5F136 Safari/525.20)
Can you try putting it on the charger or connecting it to the computer to see if that helps?
Or, you can hold down the home button and the sleep button at the same time until the iPhone shows it is rebooting with the Apple logo.