There are two possibilities. One, tourmaline can cleve pretty easily, especially if it had a fault line there to begin with.
Second, it may have done some heavy work protecting or grounding you... ie. it got overloaded with energy, and split in half. I've had this happen before... though it was green tourmaline, not black.
If the energy seems distinctly off for some reason, then it's probably the latter. A stone that had a fault line in it in the first place won't feel that different if it just split.
My advice is to find a piece to replace it and let it rest for a while... then decide if it's still yours. Sometimes, the only thing to do is leave it somewhere (if it wants to find a new owner) or bury it.