Ubuntu server stops working after a while
 

After around 4-5 hours, my ubuntu server running plex media server stops responding. Here's all the server does when crashed:

-the server screen stays black with no command line interface displaying.
-keyboard is not responding/seems completely cut from power
-cannot access server via ssh
-plex server doesn't work anymore like all other services
-Computer still runs but can only be fixed by holding the power button pushed to force shutdown the computer and then reboot.

Does this happen no matter what you are running on the server?.
Is this a recent problem? Has the server been working ok previously?
Have you made any changes recently i.e. added or updated software.

what ubuntu and plex versions are you running? Is this just a server or are you running any player applications? What is your hardware configuration i.e. how much ram etc?

If it crashes consistently after 4-5 hours it could be a hardware problem like overheating but also could be caused by running out of memory.

Im using plex Version 0.9.11.4
The hardware I'm using are all pre-assembled by HP. The model is HP Media Center PC m7680n.
The problem is not recent. Ever since ive install ubuntu server 14.04 LTS the problem exists.
The crash occurs at random time. This night the server ran 9 hours without crashing.
Plex is the only services running on the machine.

Random crashes can be interesting to troubleshoot. I might suggest stopping plex and other unnecessary services and let the PC run for awhile to see what happens. Other things you can do is run a memory check from live CD.

I suspect that this is a hard crash but you might want to look at the logs for anything. Also check the output of the dmesg command to look for any errors.