if you got blank screen (black) then you dont have to press'n'hold any button. It will recognise it as unknown usb device. And all you have to do is install the drivers
again! Just go to the control panel, select this usb device and choose the rockusb driver. Then you will be able to upgrade firmware again. And dont worry if the speed of upgrade will be very slow. After few minutes your player will be healthty again
All you guys have to do is to reinstall the rockusb drivers. Becouse system sees the player as another (not the same) device in each situation. If the black screen, reinstall, if another firmware REINSTALL.