Open your case and look for a smallish battery. Pull out that battery and wait 5 seconds. this will reset your cmos to defualt settings. Then give it another go and if you get a bootup screen then you should thank your lucky stars. If not…
can you try your monitor on a different system, if it works then your monitor is not at fault, so then we need to try a different grfx card, to find the device that is at fault, possibly try removeing & reconnecting your grafx card (assuming it’s not onboard VGA) & just double double check the VGA cable has a good connection, also is the monitor receiving power