It is definitely possible as Sims has said that you have enabled tablet mode, by going to the bottom right corner click the chat box icon, and make sure nothing is set to hidden, or tablet mode. This can really be your culprit in this situation. Also for more knowledge, dual screen setup is when you have two monitors set up side by side on the same computer, sometimes your computer detects a "Ghost Monitor" or a monitor that doesnt actually exist and set it by default on accident. The more you know!
It sounds like you inadvertently turned on tablet mode. In the bottom right corner next to the clock, click what appears to be a chat box looking icon. It will bring up a menu on the right side of the screen. Towards the bottom of that menu you will see various buttons. Find the one that is labeled tablet mode and click it.