It might be out of battery, but the pitcher still works.
The indicator has a non-replaceable, sealed battery. After several years of use, it will stop working. You should continue to change the filter regularly based on the gallon rating (40 for standard, 120 for LONGLAST) or when the water taste changes.
First, try to activate it. Open the fill lid completely for 5-8 seconds while filling the reservoir to the Max Fill line, then close the lid. A blinking green light should confirm a good filter.
If there's still no light, the battery is likely depleted. You can use the pitcher without it. Just mark your calendar for filter changes.