PC won't start

Hi guys,

Short and sweet; a few days back my PC - once the start button was pressed - would start loading for a few seconds then go dead.

I was told I needed a new power supply unit, so purchased one. I’ve connected it up, but still the PC will not start up. Its completely dead. I’ve used different plugs to check, and I don’t really have a Plan B.

Does anyone know what else could be causing the problem?