My iphone is black screen i can't get it to boot back up

[iMore Question]

Hi i can't get my phone to go into DFU mode, My laptop beeps each time i try but nothing happens, all that the computer say's is
the usb device isn't recognised and i cant get any further than this point,

Please help

 

[zerog46]

Have you plugged it into a charger for at least 30 mins. Then tried both buttons.