I'd suggest you check power supply and front panel connections first, and then try to boot the system with minimal configuration (1 CPU, 1 DIMM). Are the fans spinning up?
One of the 2 systems when I came in this morning the LED had changed from blue to green and the sytem now works. the other system still just has the blue led on.
This seconf system I have had all of the DIMMs and CPU's out and tried putting things back in one at a time and still the same results. As soon as I insert the power cord the blue LED comes on and nothing happens (NO Beeps, NO Diag LEDs, No Fans).
Now is it possible for you to switch hardware to identify the faulty part, since you have a good working system? I'd suggest start with power supply, followed by CPU and motherboard.
This applies to me too.
S5520 crashed, now blue status and ID led's plus buzzing with power-on.
Already changed the PSU, same.
Alternated the memory, same.
No memory, same.
Detached USB and fans, same.
More details on my profile/task and/or the attached text.
intel_S5520.txt.zip 355 bytes
Swap CPU and motherboard to test.