And if it is really frozen you can do a force stop.
The manual:
To Force the Device to Restart or Shut Down
Press and hold down the power key
and the volume up key at the same time
for 10 seconds.
Your device vibrates once. Depending on whether you wish to force the device to
restart or shut down, proceed as follows.
If you wish to force restart the device: Release the keys at this point. The
device restarts.
If you wish to force shut down the device: Continue pressing the keys. After a
few more seconds, the device vibrates three times and turns off.
