It could have gotten wet, and then spoiled. As you well know, at low pH (acid) bacteria are inhibited, but fungi (your brewers yeast and common molds are examples) can thrive. And thrive they will, even in moist powders that seem noxious at high concentrations. Citric acid is a great nutrient for all living things - plugs right into the metabolic pathway called -- google for it - the Citric Acid cycle.
You can save as much of the bottom portion of the bag as you can -- whatever is still crystalline and dry. Otherwise, it's back to the supply shop for you to replenish your older supplies.
Chemistry is my job, winemaking is my hobby.