To power the Pathfinder 320 off, press and hold the power button. If holding the power button for 5-10 seconds before releasing it does not power the unit off, it may need to be reset manually.
You can reset the Pathfinder 320 by disconnecting the replaceable battery. Please ensure that the power cable is disconnected from the speaker. Once the unit is not plugged into power, you can access the battery by removing the 8 screws from the back panel. If you disconnect the battery for 10 minutes and reconnect it, it should power up normally.
For detailed instructions on accessing and disconnecting the battery, please see the following article: How to Replace the Battery in an ION Portable Speaker : ION Audio