Hp Pavilion dv6 1110ax goes to sleep mode and shuts down. What might be the reasons?

Hi!, himanshu here,

                                       I have been using my hp pavilion dv6 1110ax laptop and I have fresh installed Windows 8.1 Pro genuine with all the original software drivers for this laptop. It has AMD Turion X2 64 processor, ATI hd 4650 1Gb Graphics card and 4Gb ram (3 Gb usable).

    My problem is that when I runs my PC after about 20 minutes the laptop screen darkens and the system goes in sleep mode and shuts down. When I press the power button system starts from the same memory where it slept before shut down. So, plz guide me what may have been cuasing this and how to over come this fault.