If your device is frozen, trying to move the mouse does not work, can't open or close tabs, etc, you will need to hard power it off. To do this press and hold the power button until the device fully shuts down. The screen going dark does not always mean the device has fully shut down.
To ensure that the device fully shuts down, press and hold the power button for at least 20 seconds. If you aren't sure how long to hold it, say the alphabet twice in your head! Then release the button. Wait about 10 seconds, then press the power button to turn it on.