Its difficult to say at this stage. How often does it happen. Does it happen randomly or at specific times. Does the phone get hot before it switches off and on again.
You could try booting into safe mode and see if the problem still exists.
Press side power button until power off icon appears, long press on power off icon, tap on the safe mode icon.
Your phone will restart in safe mode which essentially disables any 3rd party apps you have installed. If the phone doesn't restart unexpectedly anymore it's more than likely down to a 3rd party app. You would need to pin point the exact one yourself by a process of elimination.
Next you could try wiping the cache partition. This could help especially if you've recently installed the new UI 6.1 update
You will need to plug into a working device or pc/laptop.
https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/There's also a handy diagnostic tool in the members app under the support tab on the bottom of the members home page. Please run diagnostic on all items and report findings
