I have a Presario 700 which went dead screen on us. The LED's come on and I can hear the fan and harddrive running. I connected an external monitor and still nothing. I did the full power down with no battery or AC and held the power button 15 seconds, zip. I unseated the memory and powered down again, nada. I disassembled the machine and reseated the processor, still nothing. Any help? What next, a new Motherboard? It was replaced once already for unrelated problems. VERY frustrating.

If it's a desktop:
If you have a video card, try taking it out and connecting to integrated graphics.
Otherwise:
IDK.

It's a laptop. I had a friend look at it and he said it's the motherboard video chip. I'm going to try to reflow it and if it doesn't work I will replace the computer. It's been nothing but trouble every since new.

have you did the full power down with no battery or AC ?

I tried all the diagnostics then sent it to a friend who is a repair tech. He said the Mother Board is bad and the computer isn't worth spending the money on (AGAIN). I'm going to take it apart and try reflowing the video chip. Maybe it will work, maybe not, but all it will cost is a little effort.