Sony KV-40XBR700 I have a Sony KV-40XBR700 TV. The problem is that the TV screen is black, but the volume is ok. Once in a while the tv will turn on, but once we turn it off than it wont work for a couple of days. What seems to be the problem?

If the display seems normal when present, you may have what is called a 'cold solder joint,' possibly in the area of the horizontal power supply. This can be caused by normal heating and cooling of a particular spot on the circuit board that finally fractures the solder and making the joint thermally sensitive.
Most component failures, even when intermittant, would affect the display in some obvious way such as distortion, contrast, brightness, etc.