Apple logo showing but not switching on.

My iphone switched itself off earlier and then the Apple logo appeared on the screen like its about switch back on but nothing happened, the apple logo continues to appear then fade out. I've tried to reset the phone by holding in the buttons, I have tried disconnection the battery for a short while but still no joy so I hooked it up to my computer to try putting it into DFU mode but it's not being picked up by iTunes.

Not sure on what to do next so any help would be appreciated.

Thanks S2.