When I turn on my computer it began beeping g 4 times continuously
When I turn on my computer it beeps 4 times continuously (beep, beep, beep, beep stop then beep, beep, beep, beep). The screen would remain black. At times , the computer would come on at startup I would login and start using it, then 20 minutes or so, it would start beeping and the screen would go black. Please, help me with this for I am desperate. Your response would be so appreciated.