Am a TV technician I have a TV set that displays a picture half the screen what could be the problem the picture displaying at the bottom with the top dark screen.

Is this a CRT type TV or a flat panel TV?

If you are a TV technician then you should already know the answer to this question. If we are talking about a plasma and the screen is split top and bottom, and the bottom is completely black, then you should know this is the y-buffer.

If this is an LCD and there is a horizontal band where it is very difficult to see (no backlight in this horizontal band), then you should know it is the backlight in the panel.

However, as smoothy pointed out, we can't help you until you tell us the make and model, or at the extreme least the technology. Which is, again, something you should have known if you were a technician.