I connected my macbook pro through to a VGA screen. The desktop on the laptop screen changes as if the external display is used, but I see nothing on the screen. The screen is on, I haven't changed anything with its settings since it last worked. I tried playing with it a bit, but still nothing is shown. If I remember right, it even says "no signal" or something similar.
On another external display, everything works right (same laptop, different screen, different adaptor).
I am thinking that perhaps the problem is with the adaptor, but I want to know what other reasons could be for this happening.