There is a very typical problem with digital camera of all black screen display
when you are trying to find front view images in lcd display in camera mode.
However,in play mode you can still see your earlier captured images without any
trouble.The black screen appear in camera mode because of lens shutter not opening.
Of course if you look at the lens cover from outside,it will be found open but actually
this external cover is only for dust and other possible external demage like scratch on
lens surface,finger print etc.protection.Inside the lens mechanism there is light barrier
called IRIS which controls the amount of light to be passed to convert light energy to
electrical energy.This light energy then get converted to electrical energy by
CCD and finally this signal is sent to lcd display as image.
So if this problem appear only dark screen is visible as no light can pass through IRIS
means internal light barrier not open.
There is a possibility of CCD failure also.As already said CCD converts to electrical
singnal.If it is a CCD failure,it's need to replace CCD imager.
Whatever may be the cause ,to fix this problem you have to replace either complete
lens unit or CCD imager by expert service personal.