What happened is heat damage. Yes, your hair can lose its ability to curl and will remain permanently straight, and damaged. This happens when you use heat that's too high – it doesn't have to happen over time. One incident with a too hot curling iron, flat iron or a any hot tool and this could happen. Even if you wear your naturally curly hair straight all the time, this isn't a result you want because the hair has been fried straight and now is not healthy.
The only solution for the damaged hair is, unfortunately, to cut it off. If you have several damaged sections all over, you'll need to get rid of them, which means an overall cut. If you're trying to grow your hair longer, this is probably the last thing you want to hear, but it's necessary for the overall health of your hair. Once you cut off the damaged parts, you can start or return to a healthy hair regimen. Keep in mind that the new regrowth will be curly and healthy.
There are many hot tools with a controlled heating button. It will make you adjust the heat yourself. I also recommend a heat protectant for your hair before the heat. I have been a hairstylist since 1999 and as a professional I recommend weekly deep condition. I personally straighten my hair every two days and my hair is still curly. Hope this information helps and visit back soon.