Can your device stay on for about five or more minutes before it automatically restarts again? If it's shorter than five minutes, you need to bring your device to an Authorized Samsung Service Center immediately because this might be an indicator of a hardware issue.
If the phone stays longer than five minutes, you need to check if you accidentally activated the Auto-Restart feature of your device. To check if the Auto-Restart feature is turned on, go to Settings > Battery and Device Care > tap the three-dots menu and select Automation > toggle off the Auto Restart at set times option. If your Auto-Restart feature is turned off, you can also check if your internal storage is full or nearing its maximum capacity. If you find your device nearing its storage's maximum capacity, you need to uninstall apps that take too much space or transfer personal files to an SD card or other storage apps.
If this problem started after doing a software update, try wiping the cache partition of your device. Wiping the cache partition of your device is almost always the fix on all the issues you encounter after doing a software update. You can watch this instructional video on how to wipe the cache partition of your phone: https://www.youtube.com/watch?v=OqsSslIOSyA
