A force restart is a widely used workaround to fix many device issues like when it becomes frozen or not responding. Doing so will turn your device off and then back on. It's a safe process as it won't affect any data stored on your device.
To force restart your Galaxy Note 5 and other Samsung smartphones with non-removable batteries, just press and hold the Volume down and Power/Lock keys simultaneously for about 10 to 20 seconds. A simulated battery disconnect is carried out during the process.
Your phone must be charged to a minimum of 5 percent because your device may not power on after the restart if the battery is below that percentage level. In case, your device is completely drained, you need to charge it for a minimum of 15 minutes preferably through the wall charger supplied with it.