I have one toe nail on my big toe that is cracked horzontially in two places what could this be?

Beau's Lines: Beau's lines can result from any disease process that is severe enough to affect the growth plate of the nail. Trauma may also be a cause of beau's lines or nail ridges. The timing of the disease can be estimated by measuring the distance from the beau's line to the nail bed.

A cracked nail: Perhaps it was injured? Sometimes, it is as it appears, nothing more. If you can't recall any injury keep in mind a fot moving in a shoe sometimes is trauma enough.