Why won't my new video card work?

Bought dad a Gigabyte Radeon 5450 graphics card for Christmas. It’s not the first time I’ve installed a graphics card before, and I did it exactly the same way as last time. Everything seems to be working, however the screen is blank and the operating system does not boot. The monitor goes straight into power saving mode.

Does anyone know what could be happening with it? I’ve double and triple checked the connections. When the cable isn’t connected the monitor says “check signal cable” but when the cable is connected and the system on, the monitor goes into power saving mode.