Basically the ride was having technical problems all day, they were sending around empty carriages and one got stuck. For some reason (it's looking like this was mainly human error now- someone's going to get fired!) they sent a carriage with people round and were unaware one of the empty carriages was stuck at the bottom of a loop- which the carriage full of people then smashed into at 50mph. Luckily no fatalities but some pretty nasty broken legs for 4 people (the ones in the front row who took the most impact I believe) and the park has now been shut since, whilst investigations happen.
The ride has been pretty faulty since day one- it has been shut a few times, most notably when previously 2 bolts appeared to come off mid ride along with two outer wheel covers another time- which hit two riders. Apparently they went the cheaper route in constructing this ride, that was rumoured before it even opened! It's showing now!
Side note- Alton Towers is in Staffordshire, a good 150 miles north of London. Not too far away if you were in the states- but when England's so small that's half the country away!