I've smoothened my black hair with some product, and it was wonderful. And 1 month later my hair became damaged and not healthy. So, I want my old hair back which was black and healthy !

There are a lot of products out there that restore moisture and repair damage. I like DeLorenzo Instant Restructorant but there are a lot of similar products. You just rub it through your hair in the shower like conditioner, leave it for a few minutes and rinse, it instantly makes your hair feel 100 times softer.
I used it when I bleached my dark brown hair to a pale blonde, my hair felt like straw but this fixed it straight away!