Android is notorious for freezing (commonly called hanging) frequently. In hanged state phone fails to respond to any touch or button input. When our phone becomes unresponsive (no function or button working) what we normally do is pull the battery out, then re-insert it and then power on and done! However we can’t do this in phones with sealed batteries. Most current high-end (and even mid-range) Android phones have non-removable batteries. Considering this fact phone makers have incorporated alternatives ways to force restart (also called cold reboot or soft reset) their unresponsive devices. It is a simple but effective method to refresh the OS and solve most minor software problems.
Here is how you can force restart (soft reset) a frozen Samsung Galaxy S8 /S8+
Press and hold Power and Volume Down button until the phone reboots. Usually it takes 20-40 seconds to force restart the phone. If nothing happens even after holding these buttons for 45 seconds then release them and repeat the process once again.
This method works in most “frozen phone” cases. However if it fails, there are other options too to solve the issue like rebooting via recovery or hard reset the phone etc.