When you are having trouble, it is sometimes necessary to reboot your iDevice to get things working. First try a memory reset. If that does not work, a soft or a hard reboot may be necessary.
Soft Reset: (like hibernating a laptop): Long-press Power button until 'slide to power off' slider appears, slide to power off. Data in memory is restored at boot.
Hard Reset: (like powering a laptop off, then on): If your device has a Home button: Long-press Power button and Home until you see it start to restart with Apple logo, but keep holding Power and Home until screen goes black and stays black. Then restart with power button. For devices with NO Home Button: Quickly press and release the volume up button then quickly press and release the volume down button then press and hold the side power button until the Apple logo appears.
See also: Memory Reset
(thanks to @Ocelot and @aplourde and @marmakin)