Fortunately, a little love is all it usually takes to revive an ailing starter.
Experienced sourdough bakers know that even a healthy sourdough starter can occasionally lose vigor—either because it hasn’t been fed regularly or simply for reasons unknown (which can, and does, happen)—leaving it unable to leaven a loaf of bread. Fortunately, a little love is all it usually takes to revive an ailing starter.
HERE'S WHAT TO DO: Feed 1/4 cup (2 ounces) starter with 1/2 cup (2 1/2 ounces) all-purpose flour and 1/4 cup (2 ounces) water twice daily (approximately every 12 hours) and let it sit, covered with plastic wrap, at room temperature. It will be ready to use again when it smells pleasantly yeasty and sweet (rather than sour) and doubles in volume 8 to 12 hours after a feeding.
Sadly, this doesn’t always work. If the starter doesn’t revive at all after a day or two of feedings—it doesn’t double in volume even more than 12 hours after a feeding—it is probably beyond saving, and it’s time to start a new one from scratch.