If your Samsung phone doesn't have a touchscreen or other features that allow you to turn it off via software, the next step is to perform a hard reset using the physical buttons on the device. This will remove all data and settings, so be prepared to restore from a backup or factory reset.
To do this, press and hold both the Power and Volume Down buttons simultaneously for at least 10 seconds, until you see the Samsung logo and then hear the boot sequence start up. Release the buttons when you see the screen turn off.
Please note that a hard reset will erase all data on your device, including contacts, photos, and apps. Make sure to back up any important files or data before attempting this procedure.
Alternatively, if you have a Samsung phone with a removable battery, you can also use it as a last resort. Remove the battery and disconnect it from the phone for at least 30 seconds to fully power down the device. Reconnect the battery after 30 seconds to turn the phone back on.