I agree with J.Hidalgo and MBENZ, usually the cause of a black LCD is due to it having been broken by being hit by something.

An LCD (Liquid Crystal Display) is made of two layers of thin glass with the liquid crystal inbetween. A 'plastic' polarising filter layer sits above all this to create the blackness - by preventing the reflection of light from the display's refelective (silver/white) backplane.

Crack the glass and the liquid gets squished into places it shouldn't be, resulting in either odd black blobs or complete blackout.

The LCD will also turn black if enough heat is applied to it, but will usually return to normal once the display has cooled down again, so it's worth making sure the display isn't getting too hot before you go replacing it!

Just to further 'confuse' the issue though, some LCDs are intentionally black, created by fitting the polarising filter back-to-front from normal, the display is then lit from behind and the light shines through the whitened segments instead. So a 'black' LCD may be caused by the backlighting bulb having burnt out.

How do I know all this? I have a degree in electronics engineering!

James, UK