I have lost all my hair before.
To be honest, mine did not come back the same.
This is due to specific nutritional deficiencies and can happen even at higher/"healtier" weights. You should address this with your medical doctor, or nutritionist if you have one. Take all vitamin supplements you are prescribed. Some people have good results from biotin supplements, available in vitamin section of grocery/pharmacy store, but i have not personally. Please do tell your doctor or any other prescribers about over the counter supplements you add yourself.