Having a device become unresponsive can be a frustrating experience. Just over the weekend I was testing an Windows Phone 8.1 device, answered an email, and sat the phone back down. A few minutes later I tried to check my email, but found the device to be unresponsive. No matter what I did I couldn't get the screen to turn on.
Per Microsoft, to soft reset any Windows Phone device running Windows Phone 8 and above (including 8.1), you simply need to hold in the power and volume-down buttons at the same time for roughly 10 seconds.
Your device should vibrate when the reset occurs, letting you know that you can stop pressing the appropriate buttons.
With more and more devices shipping that lack a removable battery, manufacturers have implemented a button combination to reset the device in instances like this.
A soft reset doesn't delete anything from the device. It simply provides a means to resetting the device that's frozen.