correct diagnostic method for black screen

[mark english]

hi i have an old Sony vaio laptop VGN FS215B and have recently had the screen go black. it seems to be booting up okey ie fan going etc but no light comes from monitor. it is so difficult not being able to see any details on screen.
i have tried xp original disc but cant replace op sys still no light or cursor. i have also tried to use other screens to see whats happening but no good.
no beeps just normal noises when its booting up.
i have been told it could be back light needs replacing. its like a tube that can be replaced.
also heard about possibly pc thinks lid is down and its in hibernation. this is where you can solder and bypass switch.
anyway is there a method of things to try and rule out  before i purchase a new light.
i would be really interested in learning if the black screen problem with pcs in general can be diagnosed  and things ruled out. any ideas thanks....

[JohnK]

I'm kind of confused by your post, when you connect an external monitor, does everything display fine on that?

[mark english]

i have plugged monitor  of laptop in to my  home computer and no picture came up.

[Computer_Commando]

i have plugged monitor  of laptop in to my  home computer and no picture came up.

Not possible, you have this backwards.
Don't you mean this?
"I have plugged monitor of home computer in to my laptop and no picture came up."