We have a Samsung TV and for some reason there is a black thin line going from left to right of screen.

it has turn up in the last couple of days and its goes straight across the screen so any ideas on why its turned up and can it be fixed?

More info required.. model number?

The model number is SA40F6500 and screen size is 40in.

Usually lines on the screen are caused by a duff LCD panel, but they are more commonly vertical than horizontal.