Well try and isolate your hardware, either removing all or a portion of the new items that you have installed and see if that makes a difference for you. At some point if it starts working again you can begin adding pieces of hardware back in one component at a time and testing to see if it comes back up.
I attached a picture and listed below what i believe is in there:
A - Circuit board which checks if the case is intruded, if the fans are running, if all the power units are operational etc. beeps if something's wrong (which it doesn't do). The led display next to the powerplug on that board stated 'everything ok' by displaying a symbol that means that according to the manual.
B - Fans that cool the attached disks. There's one SCSI disk attached, which came 'off-factory' with the machine.
C - Processor and fan.
D - One DD2 400Mhz 1GB memorystrip.
E - The small connectorstrip for hdd led, speaker etc..
F - More space for memory (28GB in total).
G - Place for optional Processor.
H - Power units, 3 in total.
I - DVD Rom player, IDE.
J - SCSI Disk, 73GB Hitachi Ultrastat 10.000RPM
K - SCSI and IDE connector onboard.
L - Onboard video.
So to give you an idea of what i have done;
- I took out the RAM chip, and placed it somewhere else, no difference.
- I took out the RAM entirely, system complains with beeps, no screen, no other difference.
- I took out the processor and placed it together with the fan in the other processor-slot. No difference.
- I took out all 3 power units and put them in back in different places, no difference.
- I took out the disk and moved it around in the other four possible slots for scsi disks, nothing happened.
- I added a video card, thinking maybe onboard video is the problem, nothing happened.
To the right of B, is where the SCSI termination is going on. I'm thinking it could be there somewhere. But the strange thing is that this machine booted fine, and then just stopped at a given day. The last thing i did was install ubuntu server 64bit, which ran once.
I read somewhere that some servers (proliants i think) require certain type/brand of disks. Could Ubuntu have changed something on the disk to make the server think the disk is not ok? (though it doesn't beep/blink or do anything to state that).
Ok, long story, just wanted to give you some details so you may possibly have some other ideas.