Try a antifungal
It is hard to tell what the cause may be.
If the crack extends to the base of the nail under the skin, the crack may be due to nail root trauma. This will probably not gow out normally. If it is partially cracked, it may be an injury or fungal. In this case i would probably treat with a topical antifungal just in case. It can't hurt. If concerned, have it evaluated. Dr l.