The indicator runs on a non-replaceable, sealed battery. After several years of use, it will eventually stop working. The good news is your pitcher's filtration function is completely separate, so you can keep using it.
You'll just need to remember to change the filter manually on schedule. For a standard white filter, that's every 40 gallons (about 2 months for an average family). For a blue LONGLAST filter, it's every 120 gallons (about 6 months).
Without the indicator, it's extra important to track your usage. A simple calendar reminder can help. The pitcher itself will still filter water just fine indefinitely.