A mistrial is declared when the trial is unable to continue due to some kind of error or issue, like a jury that's unable to reach a verdict or a piece of evidence that's been mishandled. It's not the same as an acquittal, where the defendant is found not guilty, but rather a way of saying, "Hey, something went wrong, let's start fresh!" And trust me, it's not exactly what you'd call a rare occurrence - mistrials happen more often than you'd think!
In the Clancy case, a mistrial was declared, which means that Lindsay will have to wait a bit longer to find out what's next. Meanwhile, she's still hanging out at Tewksbury Hospital, waiting for the prosecution to decide how to proceed. It's like being on hold with your cable company, but instead of waiting for a customer service rep, you're waiting for a team of lawyers to figure out their next move!