What are possible reasons for eyebrow and eyelash hair loss and are there any cures?

When I was about 11 or 12 I lost all my eyebrow hair and eyelash hair. The doctor said it sometimes happens with children my age and they would grow back. I am not in my late teens and my eyebrow or eyelash hair have not grown back fully. I have hair on my eyebrows but the hairs are very tiny and very few of them. It is the same way for my eyelashes. What are possible reasons for why my eyebrow and eyelash hair fell out? Is there anything that can help them grow back?