PC Boots to no monitor signal

[Arjahn]

So me and my buddy are currently stumped about this. Last week, he fixed up my PC with a new motherboard and everything's been working absolutely fine. I ordered a new graphics card, which should be compatible with everything, and install it. After this, I boot the PC, fans and LEDs turn on, but "no signal detected" pops up on my monitor. I tried removing the graphics card and booting, but the exact same thing happens. Any idea what might be causing this?