Even iOS apps sometimes misbehave – they can crash, crash, or stop working. If you're new to iOS, or haven't done it before, you might not know how to quit an app (instead of just swiping it off your screen). Here's how to quit an app using iOS 14:
Unfortunately there is no way to close all your apps at once, if you want to you will just have to swipe them one by one.
If for some reason swiping the app doesn't fix the problem, then turn off your phone by pressing and holding the side button and one of the volume buttons until cursors appear. Swipe the one that says "slide to power off" to the right. (If you have an iPhone with a Home button, instead press and hold the side or Sleep/Wake button.)