I had this problem this night. Strangely, the solution that worked for me was really simple and stupid, a solution that is not found in any of the other answers.
I had dusted the server a bit and moved it a bit around. Dunno what I must have done, there goes the amber light when I power it on. And the Server doesn't boot. Had searched a lot on the Net, with all kinds of possibilities, all of them worrying and discouraging. I was about to declare holiday for my users, as I live in a small town and repair isn't easy to reach. And then I came across this post.
http://johnlamansky.com/tech/a-blinking-orange-power-light/
Gave me a simple solution: Turn off the spike buster and turn it on back.
What simple stuff. Yes, I turned the spike buster off and on, and bingo, all eez well.
EDIT : Well, by the time I next booted up a few times, the amber/orange light blinking had gone away, but the server wouldn't boot still, would stop after checking of DIMM slots. The same thread above, where multiple solutions are suggested and copiously acknowledged in the comments, came in handy. It was my USB keyboard. I successively disconnected my USB keyboard (yes, Dell), USB mouse (Dell) and my network cable, to check which one was the culprit.
I disconnected the USB keyboard and all is fine again. btw, it's okay to connect the USB keyboard after the server boots up.