Did you just loose access to another civ or city-state's iron? It may also be possible that a barbarian unit is blocking either your iron or even their iron, or they may have had their iron reduced by sabotage and it needs to be repaired.
From a technical standpoint there could be a corrupted file, and to resolve that you can always verify local files under the game's properties in steam. Do it until it says "All files successfully verified."