Press and hold the power button for two seconds to turn off the system. You'll see an option to "Power Off", please tap this. If the system does not turn off or you can't see the "Power Off" option, press and hold the power button for more than 30 seconds until the screen turns off. Wait for a few seconds, then press and hold the power button for five seconds to turn on your system.
With PlayStation Vita System Update v1.60 and later, you'll see a "Safe Mode" menu when your PlayStation Vita restarts after being turned off this way. It's completely normal for this screen to appear and you can restart your PlayStation Vita normally by choosing option one.
If you haven't already, you should update your PlayStation Vita's System Software to the latest version. You can update your PlayStation Vita by: