http://www.bustle.com/articles/3089...de-down-leaving-riders-terrifyingly-suspended One of the first google results for 'coaster stuck in loop.' Enter needs to do research on the things he talks about.
Well, this is the guy that insisted that whales don't eat plankton because Finding Nemo taught him they eat krill only, don't expect research from himPower could go out, the track could be fucked up due to wear and tear, a blackout could happen. A roller coaster getting stuck isn't an impossible task. Hell there is a website dedicated to the fact that amusement park rides fuck up all the time.
http://www.rideaccidents.com/