Power cycle the device (restart the device)

Android

Most Android phones can be restarted by holding the power button, then selecting “restart or power off” and restarting from the power options menu. 

Note: Newer Samsung devices require you to hold the power / side button and volume down to make the power options menu appear.

iOS

Step 1: Press and hold any of the volume buttons and the side button at the same time until the power-off slider appears.

Step 2: Drag the slider, then wait 30 seconds for your device to turn off. 

Step 3: To turn your device back on, press and hold the side button (on the right-hand side of your iPhone) until you see the Apple logo.