HD 7970 no monitor signal
I have a prebuilt computer and wanted to upgrade. I have a 550W psu, and I bought a used Sapphire Radeon HD 7970 Vapor X 3gb GHZ edition. I put it into my pc, started it, heard the windows noise, the card lights up and fans spin, but the monitor is not getting any signal, though it is definitely plugged into the wall and the PC. It just says DVI: No Signal when I try to get it to work. I only have one 8-pin to Molex cable, though there are to connectors, as the box was missing the other. Is it because of this or another reason?