Lentils, beans, and grains continue absorbing liquid after cooking. A soup that was comfortable to eat yesterday can emerge from the refrigerator as a thick paste.
Reheat the amount you need with a little water or suitable stock, stirring until hot throughout. Add liquid gradually until the texture is right. Only then taste for seasoning. If you salt the concentrated mixture first, the later adjustment becomes harder to judge.
Water is often the better choice when the soup already contains plenty of stock or salty ingredients. A splash of plant cream changes richness as well as thickness, so it is not a neutral substitute.
Once the soup is spoonable, a fresh herb or small squeeze of lemon may make it feel newly finished. Those additions should complement food that was stored safely; they do not make old or mishandled leftovers suitable to eat.