I had the following problem on the weekend
Computer worked fine,
put it in mame cab
Computer shat itself
Pulled the whole thing apart, vaccumed it all tried it with ram, without ram etc... nothing
About to give up, pulled off custom rewiring i did to put the reset and power buttons on the front of the cabinet
This worked
Looks to be almost like the reset switch was staying on, not allowing the compuer to boot up
I had many similar problems, HD worked, CPU fired up, nothing on screen
Not sure if this helps
Cheers ginno..