My iphone 4 wont turn on unless plugged in...

My iphone4 wont turn on unless its plugged into the wall or plugged into my laptop. But even when it does turn on it doesnt do anything, it just stays on the screen with the itunes logo on it. ive tried everything, ive held the home button and the off button together, and that just resets it back to the apple logo and then goes straight back to the itunes logo one. i really need some help been trying to fix it for days now.