Just built a new pc, nothing comes on the monitor.

Hello,

I just finished building my new gaming PC. I've connected everything correctly (I hope) and the computer itself powers on fine; LEDs on the motherboard and graphics card light up, etc. But when I connect it to my monitor, my monitor's screen slightly changes color but stays blank, not going into BIOS setup at all.
It's a rather old monitor that I found lying around the house, could it be a defective monitor or is it probably something inside the computer itself?