Engadget
Android stuck in Safe Mode? Here's how to turn it off

Safe Mode disables all third-party apps and automatically enables Airplane Mode, letting users isolate problematic or malicious apps; it can be entered by holding the Power Off button in the power-options menu. To exit, restart the device via the power or power-plus-volume-down keys, or tap the Safe Mode notification in the quick-settings panel, then manually disable Airplane Mode if it remains active.