Question Blank screen on HP G7 ?

[Lafa Robert]

My HP laptop is off for a month or two. It's power depended, When I switch it on the power LED is on, the mother board LED switches s on and off again, but there is nothing displayed on the screen. I tried changing RAM, HDD, screen, try resetting the bios.. The USB port too is not picking up devices like keyboard and mouse. Can any one help thanks.