People on /r/spacex have theorized (with the aid of comments from Musk) that running out of hydraulic fluid caused the grid fins to get stuck in an extreme position, which forced the rocket off course. Cold gas thrusters and gimbaling of the engine tried to correct this, but the end result was that the rocket had insufficient control authority to achieve a controlled landing.

How is the rocket going to achieve a controlled landing with no thrust? It seems to me that it would crash into the pad anyway. Or was it that they did have thrust, they were just unable to steer because of the lost pressure?

Yes I think you've got it right - they had thrust from the rocket motors, but they ran out of hydraulic fluid required to use the fans to induce more drag, so the rocket came down hard.

It makes more sense if you watch the video: https://vine.co/v/OjqeYWWpVWK

Basically, because they lost steering it came in too steep, and ended up falling onto the deck on its side because the engines couldn't compensate for that position.

Steering was the problem, not a loss of thrust.

If the rocket's trajectory is correct, all that is required is a final hard burn to negate all downward velocity a short distance above the barge deck.