It may be caused by trauma to the nail during the manicure process or the use of substandard nail polish etc. which damages the normal structure of the nail, or secondary to a bacterial or fungal infection. In this case, further operations related to damage to nail tissue should be avoided as far as possible and external or oral medication should be used if there is an infection.
Usually after removal of the cause, the nail lesion can gradually return to normal as the nail grows outwards.