Ok. First start with turning it off and on again. Then try a force restart
To force restart your device, press and hold the Volume down key and Power/Lock key on the device for 10–20 seconds. This performs a simulated battery disconnect
If that doesn't take, then you'll likely have to restore, which you'll want to backup your data
From a Home screen, tap Apps . These instructions apply to Standard mode only.
From the Personal section, tap Backup and reset.
Tap Back up my data to turn on or off .
Tap Backup account.
Tap the appropriate account.
Tap Automatic restore to turn on or off.