1) The computer cannot run without memory.
2) Without a working hard drive with an Operating System installed on it, (such as Windows), the computer is pretty much useless.
One reason a computer will start to a black screen, or to a black screen with an error message on it, is because the hard drive has failed and the computer simply can no longer boot up an Operating System.
(All mechanical hard drives will fail because they have moving parts which cause wear. And when that wear becomes great enough, the hard drive can no longer do what it was designed to do.)
Solution: Replace the worn out hard drive with a new one.