When and where did you notice it? How old is your vehicle, and are you aware of a probable cause; weather related, direct impact to the windscreen, impact to the vehicle, any work conducted on the vehicle?
Most likely a chip. The only way I've heard of a windshield breaking "spontaneously" was due to extreme heat, and slamming the door shut on a well sealed vehicle. Even then it likely already had a weak spot from a chip.
Check your insurance. Quite a lot cover glass 100%.
Is it a clean crack or is there an impact point somewhere along it? Is it possible someone or something impacted the rear view mirror? Is the rear view mirror loose and this is the cause?
Some cracks can be invisibly repaired by a glass specialist, possibly worth a punt. In the UK most insurers will cover glass repairs for free, but will charge an excess if the windscreen requires replacing, so as above, check your car insurance T&C's.