Hear everything running but see nothing

Hello,
i have a windows xp. It started freezing about a month ago or it would turn off. I took it to a friend that thought it was out of memory. He installed some memory and sent it back home. When I hooked it back up with in a short period it started doing the same thing. He said it may be when it was freezing and it was turned off to a hard boot it may have mess up something. Well it would come on boot up sometimes it would stall and usually it would not stay on but for a few minutes. Now when we turn it on thats it you can here it but the green light does not come on and monitor says sleep mode. So I can hear fan running and anything else that sounds like a computer doing its thing but no visual of anything. Do you have any idea of what my problem could be?
Thanks so much
Noah