not full screen

I am really confused my ultrawide monitor has decided to make my sims smaller on my screen. The picture I've attached is what it used to be like the white lines are where my sims screen used to go up to and now it just stopped. Also when I start up the sims my screen is low quality even tho I've got a 240hz monitor and a high end gaming pc this has only started today and I've had this pc less than a week.