Dual screen 2nd display not working when used

hi, got a setup where i have my pc and a 5m vga with dvi adapter going to the tv across the room, so if i want to watch films i can put it on the other screen. it works but as soon as anything happens on the tv screen such as a mouse movement or a window being taken over the screen goes black then blue as though there is no input, but the pc is still telling me the screen is there and working

anyone got any thoughts, is not the tv i think as it works normally with a pc input