The smiler roller coaster had some kind of a problem that involved a car train loaded with passengers to collide with a stationary empty car. Four were seriously injured and were picked up by helicopters to take them to the hospital.
From the sound of it, the computer stopped the second train, then was overridden. (I say this because witnesses reported that the train stopped where passengers could have been offloaded. This sounds like what normally happens when the computer senses something is wrong and stops the train.) For it to have started moving again, the operator would have to tell it to go and then it collided with the other car that was in the way. It’ll be a while before they say what happened officially, but that’s my guess.
More information is available from the BBC at http://www.bbc.com/news/uk-32980354