There was probably a small unnoticeable crack before

. And someone probably hit the windshield (maybe with a wet towel) too hard (or tried to swat a mosquito with the back of his hand

) and it become a foot long crack (it could have been 2 feet long too

). If you have comprehensive coverage, your insurance will take car of it with no deductible due to the "missile from the road hitting your windshield" issue. The windshield repair company will call up and get authorization for you. In addition, some insurance companies will not increase your premium if the claim is below $500. The new windshield should not cost more than $350. If you are not sure about the increase in premium, call your agent to confirm.
Edit.
Originally posted by Pete D:
Hot windshield and cold water can crack it (or so I've been told). ...
That is correct. Usually, it happens because the windshield had a small unnoticeable crack to begin with. The integrity of the windshield was already compromised due to the small crack.