Both the rail bridge and the road bridge. A section of the road bridge is in the water and so is impassable. The rail bridge has a fuel container train on it that is engulfed in flames.
An attack? It's hard to know without evidence. But it's odd that both parts of the bridge were damaged at the same time. Not sure whether early reports are correct that a lorry exploded on the road bridge and ignited the train.
If the Russians can't use the bridge for a while, that is a huge setback for them. You'd be very nervous if you are stationed in Crimea.
http://www.bbc.co.uk/news/world-europe-63183404