What to do about burned hair?
If you burn your hair with a hot tool, you most likely have to have the burned portion trimmed. A stylist can help you find a style that works.
If your hair is burned from constantly using heat styling tools, but not literally burned off, you need to take care of it. Use a conditioning, restoring mask and make sure to use heat protection before styling.
I accidentally burned off my leg hair in a fire and now it is really short so when will it grow back?
Unfortunately, the hair that is that damaged cannot be repaired. All you can do is to let it grow out, having the burned hair cut off periodically so that eventually you will be back to a full head of healthy hair. Meanwhile, use a gentle shampoo and conditioner and refrain from coloring or otherwise chemically treating your hair until the burned areas are gone.
Hair is soft due to the disulfide bonds in the alpha keratin (a protein) in hair. Too many disulfide bonds and you have a rigid structure like fingernails but a few disulfide bonds will give a soft flexible and stretchy fiber. The sulfur in hair and fingernails is also why they smell so bad when burned.