You can become very sick. Also you can die from lack of salt.
Lack of salt, is just that, you are not consuming enough in your diet. I had a friend who ended up with thyroid problems, was a vegetarian and insisted never adding salt to food. Her doctor told her, you should not be afraid to add a little to your diet, as this is what caused the condition. That is why salt, through the ages has been considered valuable. Hence, the expression, worth your weight in salt. Some cattle have to be force fed large amounts of salt to prevent dehydration. Everything in moderation, but if low on something best to increase in your diet until things go back to normal