help me understand what's broken

when the ON button is pressed, the computer starts BUT
- the monitor remains dark (the monitor is working and all the wires to it too) - the
lights on the system unit
are not on already checked the video card (it works)
cleaned the contacts on the operational one - the
CMOS did not help the battery pulled out did not help either
And when trying turn off the computer with the ON button, it does not respond to pressing and holding the button
to pressing the reset button, there is also no response