What to do to make my hair thick again?

I had well-groomed long, thick hair, unfortunately at the age of 14 I started "experiments on hair", I dyed, I brightened myself at home, then I dyed again to a dark color. I regret very much that I destroyed my hair. So many times they have been brightened that they are simply burned. Then, after two pregnancies, they began to fall out terribly, break. Today I have little hair, short and burnt. I don't know what to do to make them delicate and thick again. Is there any good drug to buy at the pharmacy to strengthen the hair , regenerate it?