This is directly from the X-Plane user's manual in your instructions folder. I think maybe your joystick is unplugged from the USB?
How to Fly
When flying for the first time (both in X‑Plane and the real world), it’s a good idea to use a relatively simple aircraft. The Cessna 172 is an excellent choice in this regard, a fact attested to by the millions of real-world pilots trained in this model.
Before beginning, be sure you have configured your flight controls, if applicable, per the section “Configuring Flight Controls” of the chapter Configuring and Tuning Your X‑Plane Installation. If you are not using flight controls, you will have to fly with the mouse. In this case, there will be a small white plus sign (+) in the center of the screen. If only this cross is visible, with no white box around it, X‑Plane is indicating that the pilot’s “hand” is not on the stick. This means that the mouse is free to move anywhere without impacting the flight controls.
To grab the stick (and thus take control of the aircraft), click the left mouse button in the vicinity of the little white cross and a white box will appear around the cross. The mouse button should not be held down, only clicked once to turn the box on (i.e., to grab the stick) and again to turn the box off (to release the stick). When the box is visible, the pilot’s hand is on the stick and any movements of the mouse within the box will position the flight controls accordingly. Thus, moving the mouse directly below the cross will command some up elevator (causing the plane to climb) and will not impose any roll commands (which should keep the aircraft from changing its bank). Likewise, keeping the mouse lined up exactly with the cross but deflecting it to the right a bit will cause the plane to bank to the right without altering its pitch.