This is an indication that the inverter has shut itself off by:
Low or high voltage input (<10.5, >15.5 [GO-AC models <9.5, >16])
Thermal shutdown (unit overheated)
A start-up surge higher than the peak wattage of the inverter
A shorted appliance
The unit may be defective if the LED is illuminated and no appliances are plugged into the inverter. This could be caused by an extreme limit of one of the above examples.