black screen, no chime, no keyboard - Solved!

My iMac at start up went through the boot halfway then shutdown suddenly. When I booted it back up it did the same thing then went to a black screen.

I tried resetting the SMC, PRAM etc ... then realized that the keyboard was not communicating with the computer. I double checked the keyboard on my home iMac and it worked just fine, but nothing with my office computer.

Brought my office system home to trouble shoot. Checked LED lights and all but number 4 are lit. I checked all cables and connections and everything appears to be fine. The machine powers up and the DVD drive engages but then nothing happens. No boot chime of any kind. I can hear the HDD running and I removed and checked the RAM, no change.

I tried using an external monitor and hooking my computer up through target mode, my iMac and the external monitor did not find the computer because the ports are not communication? Without a working keyboard how do I speak to the computer.

I finally replaced the video/graphics card with a used but warrantied replacement unit that I purchased online. I followed every step precisely. Crossed my fingers and turned it on and nothing.

I have had it turned off for two weeks now trying to come up with my next move. Today I turned it on and still nothing. It powers on but the computer does not turn on.

Any suggestions or direction would be very much appreciated.

I just wanted to update everyone on the status of my machine. I have finally got it back up and running!!!! I am so excited. The logic board was truly the missing piece to the puzzle. I received the part in the mail saturday and had time this evening to work on it. I put the last screw in place, plugged it in and held my breath. The sound of that chime was music to my ears :0) It is working like a champ. A bit slower then I remember haha....but I think I can finally put that larger hard drive in now. That will be the icing on the cake. Knock on wood that nothing goes wrong with that. But after all that I have been through so far, it should be a walk in the park. Thank you Meyer for all your guidance and support. And I can't thank the people at applepartsexchange enough for providing me with the logic board. A good ending to a long and interesting journey.