If monitor and cable ok (proved by swapping) then graphics card / drivers at fault.
Does this Pbell have on board graphics or a seperate graphic card or both?
On board graphics disabled in BIOS? then graphics card removed (you say 2nd hand machine)
No POST beeps to inform of badly fitted card?
try resetting the BIOS (I know you can't see it)a small jumper on the motherboard or remove the CMOS battery for 30 secs will reset the BIOS to default settings.