Server 2003 SBS Crashing seemingly at random
Solution 1:
It could be a million things from a faulty driver down to hardware problems.
I would say before going any further, the best thing to try would be to go to system properties (right click on computer and click properties) then go to the advanced tab, click startup and recovery and untick the Automatically restart button and choose to keel a kernel memory dump.
Next time this occurs, you should be able to see if the problem is related to memory, hardware or driver based on what it says on the BSOD. If the BSOD does not help, You should have a kernel dump that you can diagnose (if you can be bothered!)
Of course, it is possible that it won't even reach a Blue screen of death, and if this is the case, it most likly means a hardware fault such as power.
Solution 2:
It can be a power supply issue, even if it doesn't power down. Unstable +12V can be hell for computers.
I'd burn a CD with memtest86+ and run it for a while to make sure the RAM isn't faulty. If the RAM checks out fine, I'd try to replace the PSU.
Others mentioned faulty drivers, but drivers doesn't work fine one day and crash the other (I'm guessing that the server ran just fine pre-1-year ago).
Edit: Didn't see the part about you not having a monitor attached. If it bluescreens, try my suggestions above. If it's just networking missing, replace NIC/cable/switch etc..