After delivering the payload, SpaceX guided the rocket to a floating droneship in an attempt to recuperate the rocket's first stage. The attempt was very close but no cigar, touchdown speed was OK but the rocket tipped over post landing. On his Twitter feed, SpaceX founder Elon Musk explains one of the lockout collets didn't latch on one of the four legs. The exact reason is unknown, but he speculates the root cause may have been ice buildup due to condensation from heavy fog at liftoff.
Earlier attempts to land on a floating barge in January and April of 2015 failed but SpaceX is attempting to perfect both land-based and sea-based landings. Rocket landings at sea are harder because of the much smaller target area, and because the barge moves with waves and currents.
Despite the increased difficulty of landing on a ship, SpaceX says this is absolutely necessary for high-velocity missions. When the payload is too heavy or is going too high an orbit, there's not enough fuel left to zero out lateral velocity, which makes it physically impossible to return to a land-based landing spot.
A video of the botched landing can be viewed below, it's very impressive how close to perfection this was and I have no doubt SpaceX will be able to achieve a sea-based landing on one of its next missions. I'm also looking forward to see if SpaceX can achieve its goal of making its rockets reusable, landing a rocket is one (very impressive) thing but actually reusing it will be another major milestone. If the first stage can be reused multiple times it will significantly lower the cost of rocket launches.