There's a few things to try before taking it for repair. Freezing and restarting could be down to a number of things, such as corrupted apps.
First, try wiping the phones cache partition.
Plug into a working PC or laptop or other Samsung device
https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/Secondly, try booting to safe mode. This will essentially disabled all 3rd party apps or customisations that could potentially be causing your issues.
To get to safe mode - long press power button, long press power off icon, tap safe mode icon. When booted into Safe mode, you may need to turn off flight mode as it sometimes enables itself. Monitor and check if the phone is still freezing/restarting. If this fixes your problem, then you'll need to find out which 3rd party app or customisation is causing the issue. Do this by uninstalling all and reinstalling one by one until you find the culprit.
Thirdly, try resetting by going to settings>general management >reset>reset all settings. Don't worry this won't wipe any data.
Finally, if all above fails to help then perform a backup and factory reset.
