Why does my league in game have only 1 resolution

I was play league and everything was fine and i got off to take a break then i got back on was going to play 1 game but the game was to detailed for me i could not play it was lagging and before i was playing on a resolution of somthing else but now there is only 1920x1080 so i have been playing on this desktop for 2 years straight and i dont know why this is happening someone please tell me and help me im on a windows 8.1