As a first step I would advise ensuring that all cables and connections are firmly in place, both inside tower, on and between motherboard and drives etc. and external to keyboard, monitor, mouse etc. Reseat everything including RAM and Cards.
By 'Black Screen' I presume you mean no display on monitor? as opposed to having a black screen with flashing cursor? I would have thought if the pc was crashing you would get a BSOD.