Perhaps you have unknowingly enabled a slide-over view. If so, you need to explore multitasking...
Key to accessing and controlling multi-tasking is the small “three-dot” button that is found at the top-centre edge of each App Window. Tap the multitasking button at the top of the slide-over window to expose a menu - from which you can change the window view.
More information about using multi-tasking can be found in this support page:
Use multitasking on your iPad - Apple Support
The iPad User Guide is also a rich source of information about using iPad and its many features. The Guide is available online using a web browser (such as Safari):
iPad User Guide - Apple Support