J.L.
I am going through the same thing right now. Mine is because I injured my toenail. Before I could clip it, it was really long and I accidently kicked my daughters metal bed frame...ouchie. After that it was gone. It never did turn black, it just turned like a white-ish/yellow-ish color and was dead. Now it's growing back, but very slow, and very thick. I do an OTC med that seems to be working. I just asked the pharmacist at target what she recommended.
I did some research on it when this hapened to me and there were a few things to take into consideration in my case. They said it could be a fungus, which of course could be caused by numerous things, it could also be from pedicure, more specifically when they stick that little tool under your nail to scrape all the dirt and debris out, supposedly that tool damages your nail bed slowly. They also said some medications you take in combination with sun exposure (like wearing flip flops) could cause the nail to lift (tetracycline was one) and injuries to your nail bed (dropping something on them, stubbing them etc).
I hope this info helps. Good Luck!