laptop turns on but screen is blank

I power up my laptop power comes on anboot light flashes and sceen is black after booting for 20 minutes all power shuts down

A black display can be caused by two components, the display or the graphics adapter. The best test is to connect to an external monitor, if the monitor can display then the problem is the laptop display. If the monitor does not display then the problem is the graphics adapter which in most cases is soldered onto the system board (replacing the system board will fix this video problem). The laptop display may have lost the backlight, if you shine a flashlight on the display and see icons on the desktop then the problem is the backlight.