You can't repair damaged nails, the damage needs to grow out and this can take upto 9 months from cuticle to free edge. You can use a nail strengthener (not hardener) or use a non buff Gel Polish range like CND Shellac to help support them as they grow. You could also try IBX but personally I experienced lots if breakage after IBX on my naturally thin, ridged nails.
Why are they damaged in the first place?