Question 4K tv and 4K graphics card but only 1080 being displayed

[halfitzsimons]

Hi, I have a tower computer plugged into my 4K tv. I was getting a nice 4K display but then something reset and now I'm getting a 1080 display. In Windows 10 settings my desktop display is 3840x2160 but my active signal resolution is 1920x1080 and when I switch my tv on the information banner that briefly appears tells me the resolution is 1920x1080, Can anybody help me get it back to 4K please. Many thanks in advance.