Re: Very deep groove in middle toenail???
Usually a deep horizontal groove across the nail would signify Beau's Lines, but it would also be accompanied by a discoloration in, or at, the line.
If you have not damaged the matrix in any way that you remember, this could signify something else entirely - something systemic, so you might want to seek medical advice. Ay change in the nail plate is cause for concern - especially since our nails are the first thing to experience changes if there is something wrong internally.