Why broth sets in the fridge.
During simmering, collagen from connective tissue turns into gelatin. As the broth cools, the gelatin forms a soft gel that holds water. Gentle heat loosens that structure again: the very same broth becomes liquid in your cup.
Spoon the chilled broth into a small saucepan and warm it gently, stirring as it melts. Add hot water a little at a time if you prefer a lighter cup. For a sauce, use it as it is and let it reduce in the pan.
Temperature and concentration both affect how firmly broth sets. A softer or firmer gel can be perfectly normal; texture alone does not tell you how many grams of protein a portion contains.