I used to have thick hair and now when my parents gave me a short hair cut my hair turned really thin?

so how can I grow out my hair up to my back and how can I get my thick hair back? and I use the straightener a lot because my hair gets wavy and like a little bit curly not that much so what should I do to keep my hair nice thick and healthy? I used to have thick hair and now when my parents gave me a short hair cut my hair turned really thin?
Just grow it out again. When it's all one length it should feel thicker, to keep it healthy just use a good shampoo and conditioner and take it easy with the straightener. They damage your hair.