When turning on your PlayStation Vita system for the first time, or when the console has been turned off, press and hold the power button for five seconds. When the power comes on, the PS Button lights up blue If the PS button does not light up blue, please click here to find out what to do if your PlayStation Vita isn’t turning on.
To check this you'll need to force the PlayStation Vita to turn off. Press and hold the power button for more than 30 seconds to completely turn off the system and then release the button. After a moment press and hold the power button again for five seconds to start up the system.