If you're lucky all you did was short out your power supply. If you're not so lucky, you made a feedback pulse that took out who knows how many different components on your computer.
To start with, go and get another power supply, because you've almost certainly fried the existing one if I'm understanding what you did correctly.
I think I've made a schoolboy error, as I was fitting my new graphics card, i attached two "loose" molex 4 pin connectors together, the computer did not turn on, and still won't do now. Absolutely nothing happens when pressing on switch. Any suggestions on how to get it going again would be much appreciated.