A soft reset (turning the device off then back on) is similar to restarting a desktop computer and is one of the first steps in troubleshooting a device. In most cases, a soft reset won't affect data on the device unless the battery is very low.
Press and hold the Power button (located on the right edge of the device; above the Volume buttons) until the options menu appears then release. If the display is frozen/unresponsive:
Open the SIM card slot cover.
Using the tip of a pen or a similar object, press and hold down the Yellow OFF button until the device shuts down. Do not use overly sharp objects that could damage the OFF button.
Tap Power off.
From the 'Your phone will shut down' prompt, tap OK to confirm. Allow several seconds for the device to power off.
Press and hold the Power button until the Sony logo appears then release. Allow several seconds for the device to power on.