don't hold the button down right as you turn it on- the beeps are likely a keyboard error, where it thinks your keyboard has a stuck key (due to you holding it down).
wait a few seconds after the boot-up- until you see something on the screen. Most PCs will display something to the effect of "press Del to enter setup"
However, from the sounds of things you also have no display. does your motherboard have on-board graphics? the setting you toggled may have changed the preferred display device to the on-board graphics as opposed to your nvidia card, which I presume your monitor is connected to.