How can I individually diagnose problematic equipment?

Question

Iron

Hello.
When you start the PC (old desktop, 2007), only a black screen appears.
No display on the monitor.
The speaker doesn't beep at all.
Everything is fine with the monitor. There is no integrated GPU.
I suspect that this is:
Video card: I put another used one suitable for this PC. The result is the same. There is a possibility that this video card is not working.
Bios
Motherboard: failure of the radio components or the bridge / bus itself
Power supply (although the LED on the motherboard lights up)
I don’t use this computer often, the last time I turned it on was more than 2 weeks ago and everything worked fine. There is no other suitable PC to check the operation of the motherboard / video card / power supply one by one
How can I individually diagnose problematic equipment?
I don’t want to take it to the service center yet, I want to check it myself!
Is there a possibility it's a virus?
I will be glad for any advice/links to useful resources.
Thanks in advance!

Answer 1

do you have a multimeter?
Do the fans start when turned on?

Answer 2

Thanks everyone for the advice. Pulled out the video card and memory, cleaned the contacts. Then I checked each bar individually in memtest86. Installed all the strips, now everything works.