The main cause of alopecia is inflammation. If the inflammation is temporary, then you hair loss is also temporary and vice versa. However, the bigger picture here is why this inflammation occurs. Well, alopecia is an autoimmune disease, which means your immune system is incorrectly auto set to attack the cells of your own body. Suddenly, your hair follicles seem to appear as foreign invaders to the immune system, and thus the attack takes place.
Some say alopecia is also triggered by environmental factors like stress (as you mentioned), infections, viruses, certain medications, and of course, genetics.
I think the best treatment would be anti-inflammatory medications to stop the inflammation that is promoting the alopecia, or stress-related medications if stress is the cause. You can also consider topical sensitizers, minoxidil, and shampoos for sensitive scalps. However, if it has become too late or there are permanent bald patches on your head, a hair transplant should do.
If all of these fail, your last resort would be to talk to a hair loss specialist.