EAH 4850 Problems( no video)

Hello. I am just frustrated with a problem.I have the asus hd4850top edition 512mb video card and everything worked fine until one day. i turned up my pc and nothing.. NO VIDEO BUT YOU COULD STILL hear the beep of windows booting up. and the cards fan turns on and everything. i have a 600watt power supply and pci-e adapter checked everything changed cables to new ones changed thermal paste and nothing .. also when i boot up the pc by the 4850 i can take the vga cable and plug it in to the internal and gives image perfecty looked at bios to see if pci-e port disabled but no... so thanks in advance for anyone trying to help me.. you know its hard to go back to integrated graphics.. pc specs amd athlon x2 4000 ecs elitegroup a780gm-a motherboard 2gb ddr2 800mhz 80gb hhd 500gb hdd 600 watt psu