What is your BMI? People on the lower end of a healthy BMI likely shouldn’t do extended water fasts. I think you will see the same benefits without these side effects doing shorter ones. Not getting your period is 100% your body saying “you need to eat more”.
That said, eating healthily and at your maintenance calories for a while will more than likely get it back. If you don’t see it come back in 6 months, you should probably see a doctor.