Have had my XXL/535/540 for 5 months. Today I was driving to an appointment and had it plugged into my car as usual and, in the middle of speaking directions, it suddenly stopped and went dark as if it had been turned off. Tried plugging it in to an outlet inside the house and still is dead. The green light on the bottom is on and the light on the adapter is on but the unit is dead. What's up?