Dell Inspiron 15 won't boot, 3 beeps

Hello, my dell core i3 inspiron 15 is unable to start. yesterday it at once gave an error like ( sorry your system ran into a problem we are restarting your system.......you can search online for the prob..) after that when I pressed the power button start up light or power light turned on for while and then off, after 5 seconds a series of 3 beeps started like beep.beep.beep again after 5 seconds beep.beep.beep continuously, and if I connect ac adapter then power light remains on but problem still exits. I have reseated ram,hard disk replaced with other ram and hard disk, released the static power etc but nothing happened plz help,,,,thanks