Short Answer 
Disabling the system UI on an Android device can severely impair functionality, resulting in issues like loss of home screen access, notifications, and navigation. If this occurs, users can restart the device in safe mode or, if necessary, perform a factory reset after backing up important data.
Understanding System UI
The system UI on Android manages crucial functions like the home screen, notifications, and system menus. When you disable it, you essentially stop the core visual components that allow user interaction with the device. This can lead to serious issues, including an unresponsive or partially functional phone.
Consequences of Disabling
Disabling the system UI can lead to various unpredictable outcomes, such as:
- A complete lack of access to the home screen.
- Inability to receive or view notifications.
- Potential system crashes or freezes.
- Loss of basic navigation options within the device.
Restoring Functionality
If you’ve disabled the system UI accidentally, consider these steps to restore functionality:
- Restart your device in safe mode to troubleshoot and re-enable the system UI.
- If safe mode doesn’t work, you may need to perform a factory reset to bring everything back to default settings.
- Always ensure to back up important data before taking any drastic measures like a factory reset.