I have a fingernail that partially fell off. it started with a deep crease in the nail. then it fell off right above where the crease started.

Nail Dystrophy: You most likely have median nail dystrophy. If you pick your nails, or manipulate the skin around the nails, this can cause this nail condition. If there has been significant trauma, there is scaring in your nail bed which cannot be cured. I would have your doctor write you a prescription for tacrolimus apply this medication to the skin around the nails twice a day for 6-8 weeks.

Cool! / ?Gross?: Horizontal ridges (across the nail) usually appear as a result of direct trauma to the nail / nailbed, or major life stressor / toxin exposure as the nail was developing. Often, as nails only grow a few mm/week, by the time it is noticeable, often the injury or other event is forgotten. When only on one finger, more likely local trauma as source.