How long does it take your hair to grow back if it thinned out do to excessive straightening?does it grow back?

i used to have thick frizzy hair, so like everyone else in my stupid generation i flat ironed my hari from middle school--im now a senior in highschool--my hair got so thin, its half of what it used to be. i stopped using heat products on my hair. will that ensure that my hair grows back to be thick again? WILL it grow back in to be thick? is there anything i can do to help thicken my hair again? PLEASE HELP:(