iphone won't turn on

I'm really curious is what caused this. My iphone will not turn on past the apple logo screen it stops trying after a couple seconds and turns back off. This happened when i downloaded tethering for 3g. I figured it was no big deal and when i got home would plug in and restore. But this won't work as it doesn't read it is connected. And it won't go into dfu mode so i don't know what is going on with it. When i plug it in the computer does turn it on so the connection works i just hope someone has a solution.