Screen going blank

[zulujuju]

I have a Mac Book which is about 6 years old.

The problem is when I open the laptop up the screen will be black but it makes the usual noises so I know it has come on.

If I look closely at the screen I can see it that it is on in the background.

Sometimes if I move the screen at different angles it will come on and then after a little while it will go blank again, so I have to move it to another angle.

it usually stays on if it is pushed all the way back.

Other times I have to leave it for about an hour for it to come on. I can plug it in to an external monitor and it will work fine.

Does anyone have any idea's what it could be and if it's expensive to repair.

Thanks