Hunger Lock is a state of incapacitation for Townsfolk brought on by hunger.
Townsfolk enter hunger lock when their hunger levels get too high, but there is no edible food accessible on the map.
If your Townsfolk are stationary despite having actions to tend to, and have a picture of a loaf of bread with a red cross through it flashing on their face, they have entered hunger lock.
If only a few of your Townsfolk have entered hunger lock, it is not too late to save them! Order the remaining folk to harvest some apples or pears or harvest some cows for milk. Eating raw meat is even enough to stave off starvation, but quickly building a Cooking Fire to cook some meat is also a possibility, if you can do it fast enough.
If you take too long, or if all your Townsfolk manage to enter hunger lock, there is nothing you can do. After a short while, hunger locked folk will starve and die. Next time, try preparing and stocking more food.