If your Lumia 2520 is not responding, for example you have locked the screen and it will not unlock from a sleep state. Please try the following steps:
Long press the power key for a period of 15 seconds. This should shut down the device.
If holding down the power key for a period of 15 seconds does not shut down the device. You may try holding down the power and volume-down button together for a period of 70 seconds. This will shut down the device.
Once the device has shut down, you can reboot the Lumia 2520 as you would do normally.