Ok, you didn't mention that it wouldn't even boot; Server Monitor does rely on the OS for info, so that may not help. Though if the fans spin up it's at least possible it's getting far enough into the boot process to give status to Server Monitor.
Does it have a graphics card in it? If so, what does a monitor connected show? The yellow indicator light will, I assume, only flash a code if it fails the hardware check before beginning the boot process--if it's booting partway then panicking or otherwise freezing, the light probably won't tell you anything.
Swapping out RAM (or just moving it to another slot--I've heard of slots failing) is a relatively easy thing to try, though, so it's probably worth a shot. Otherwise see if it'll boot from a system disk--might be a bad drive.