It has happened to all of us. We have been growing our nails so they will look terrific for that important big day, then the day before the event a nail tears, sometimes even lower than the quick, ooch!.
Don't worry, this can be fixed. Take the paper from a dried used tea bag and tear of a piece bigger than the nail tear. It's important that the edge of the piece is torn all the way around, no straight lines. This is so that the piece can be incorporated into the nail surface without leaving an obvious edge.
Apply a generous layer of base coat to the whole of the affected nail and, while its still wet, place the piece of paper over the tear in the nail. Push down well so that the paper is bedded into the base coat. Allow this to dry completely then apply another coat of base coat on top. When this is dry apply several coats of your usual nail polish until the paper is invisible.
This patch will last for quite a while. Apply a new coat of polish or top coat every day to extend it's life. This trick should get you through your important big day and allow the nail to grow up a bit beyond the quick so that when you have to trim the nail and it's patch it won't be sore and will look OK.