If your hair is severely damaged unfortunately nothing can bring that back to health, hair is dead so cannot permanently be repaired. Sometimes curl is temporarily lost due to dehydration but it can be permanently lost due to damage to the protein structure of the hair, so you may need to lose some length.
For hair closer to the roots that is less damaged try a combination of hydrolysed protein treatments, coconut oil and honey soaks (clean dry hair may absorb better), making sure every product in your routine is hydrating your hair, go 'strict' Curly Girl instead of shampooing, stop all heat styling.
What is your routine at present? Have you considered a hair analysis like Komaza Care to see how far up the hair the severe damage is? They can tell you exactly how much needs to be trimmed away.
Komaza Care HAIR ANALYSIS Service - Rave! - Long Hair Care Forum
2a-2c, medium texture, porous/ colour treated. Three years CG, growing out mechanical and chemical damage = breakage and very high porosity. Past armpit length heading for waist.
CO-wash: Inecto coconut
Treatments: Komaza Matani, coconut oil, Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss ultimate volume, Inecto argan
Styler: Umberto Giannini jelly, Boots Essentials gel
Experimenting with: going back to basics