Question

Since Feb 8th, when I unlock the screen lock on the device, the device restarts immediately itself(causing repeatedly restarting).

Answer
 

We have identified that this issue may occur because of certain apps not being installed properly through Google Play Store.
 

Please enter Safe mode and then try updating the apps again that have been updated since Feb 8th. This may improve the issue.
 

*Path : Enter Safe mode > connect to Wi-Fi > open Google Play Store > select account icon at top right corner > My apps & games(Manage apps & device) > 'Manage' tab on the top > tap Sort by 'Recently updated' > Check recently updated app lists > update all the apps again that have been ever updated since Feb 8th
 

[How to enter Safe mode]

 - Press and hold the volume down button while the phone is restarting, and then release the button once "Safe Mode" appears at the bottom left corner.

* Running Safe mode may remove some Home screen widgets. Please reset the widgets after taking normal restart action.