have you got the correct display drivers installed?
right click anywhere on a empty part of your desktop and select properties to bring up the display properties and have a really good look round all the display options, including clicking on any advanced buttons there may be and see if you can see anything about changing displays.
If there is nothing in there you may have to install the proper display drivers for that model to be able to use the TV output.
http://www.medion.co.uk/