depends, could be loads of things, was it overclocked?
I don't know about new ram, but the old pc133 wouldn't allow video if 1 stick was damaged/went bad sometimes. if you had an OC'd video card, it could have overheated, same for CPU and RAM.
you should also check your monitor, either use a friends PC and see if it works, or use their monitor and see if the PC works.
I'd start with telling us if it was Oc'd, and what part was oc'd, since that would be the first place to look.