Bought new hardware for PC and NOT turning on?

I bought an XFX Radeon HD 4850 a ATX 450W 12V 2.2 and 2GB of 800mhz RAM.

System: Hp Pavilion with a PCI-E x16 slot.

Micro-ATX Motherboard, the card was able to fit on the motherboard.

After hooking up everything it was fine until I turned it off and tried to install some screws to keep in in place, I couldnt.

When I turned it back on nothing. All the wires were in their correct places, I double check everything many times but it wouldnt power up.

Processor AMD Athlon x64 2.2 Ghz Dual-core

System date: 2006 Computer.

3GB RAM with x2 512MB and 1 2GB.

Why is this happening i think it fried.