Try this and it works - 50% of the time...
Remove both the SIM cards & Memory cards if any; Then restart the mobile. Without SIM cards & memory cards,
1. After Taking All Data Back-Up, Factory Reset - Settings - System - Reset Options - 1st Reset WiFi, Mobile & Bluetooth - 2nd Reset App Preferences & 3rd Erase All Data (Factory Reset)
After The Mobile Restarts, It Will Bring You To The Language Selection Screen. Switch Off The Mobile.
2. Press (also hold) power key and then press volume up key.
3. Press "wipe cache partition" volume down key (or volume up key)
4. Select "wipe data/factory reset" by volume down key (or volume up key)
5. and then press power key to start reset.
Things should be working fine.
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