Computer tower not displaying to monitor

[Crestfallen]

I've got this computer tower and it wasn't used for quite a while then when we grabbed it and tried to turn it on, it wouldn't display anything on the monitor. Whenever we turn it on it says "no signal". I know it's not the power supply as all of the lights and fans turn on, and my led keyboard lights up when plugged in to the usb. I've looked on the motherboard and nothing appears to be broken.

The fixes we've tried:
Turning if off and on again
Unplugging the power cord and holding the power for about 15 seconds then plugging the power cord back in
Reseating the ram, and making sure all cables were plugged in all the way
Taking the CMOS battery out for about 5 minutes and putting it back in
Making sure the monitor receives signal from other devices
Using a cord that has been shown to work
Tried using the onboard graphics and several video cards

Haven't tried:
Anything involving the CPU

I'm quite stuck and have no clue where to go as I don't know what's causing the issue. If anyone is able to help that would be much appreciated. Thank you.

[Johnharper241]

If your computer starts but displays nothing, you should check is if your monitor is working properly. Check the power light of your monitor to verify that it's turned on. If your monitor won't turn on, unplug the power adapter of your monitor, and then plug it back into the power outlet.

Greetings,
John

[Prométhée]

hi

@Crestfallen 

from what you explain, you have already tried everything, even the monitor with another source, and it seems to work well, correct?
The computer, may seem to start, everything works, the fans, etc
but no video signal
So you have either a graphics card problem, only original it seems that no pci card

https://support.hp.com/us-en/document/c04058427

this one ?
either a motherboard / bios problem, or other, difficult to diagnose remotely
"I've got this computer tower and it wasn't used for quite a while then when we grabbed it and tried to turn it on,"
how long, was it sheltered (humidity maybe)
Have you tried replacing it without the CMOS battery?
Not necessarily a confirmation sign, but if you remove the memory, restart it beep?