My computer is turning on but doesn't flash hdd activity or....

I currently have a computer that just one day stopped showing anything else than no signal on the monitor and the computer doesn't flash harddisk activity when turning on. It also deosn't beep any post codes.

I have tryed removing the graphic card and running on the built in graphics card. I also tryed another cable, checked all internal cabeling, and everything plugged in to the computer, but it still doesn't show anything on the screen or the hdd activity LED.

Computer Model: HP Compaq dc7600 Covertible Minitower
Monitor Model: HP LP1965

What could be the problem..? I have run out of ideas.. When plugging a laptop to the monitor it gives a image, on the monitor..