My X51 (although old) has been working perfectly since I bought it. 

Today on turning the machine on, It seems to beep-pause-beep-pause endlessly with a black screen. I have tried a few times since and once I got a code of 5 beeps but only the once. 

I have never upgraded parts nor opened the case. Did the usual googling of the problem and its a mixed bag, so I thought I'd ask the question on here.

Any ideas what is wrong? Thanks in advance