Have you knocked your tooth at all? If not then it could be just a small naturally occuring hairline striation. Iv'e got a couple on my front teeth and they look like cracks but they are perfectly healthy and shouldn't cause any problems. Quite a lot of people have them without realising.
I'm not a dentist but a dental nurse of 15 years experience, and Iv'e seen lot's of these "cracks" if you'll pardon the expression!
Try not to worry but get it checked out with your dentist especially if you have knocked it.
x