Mistrial is declared in criminal law trials when a case is dismissed prior to its normal conclusion. A mistrial has no legal force and is considered null. The judge usually awards a retrial on the same subject, with an important exception in the United States: the provision of double jeopardy in the Fifth Amendment prohibits retrial for the same offense if a mistrial is declared erroneously, if mistrial is declared against the defendant's wishes, or if it is declared because of prosecutorial or judicial misconduct.
The most common event that leads to a mistrial is when a jury cannot reach consensus on a verdict. In this case, the jury is said to be "hung" and if no agreement can be reached even after repeated attempts, then the court may declare a mistrial.
There are many other events that can lead to a mistrial, most of which involve some gross procedural error or professional misconduct. Misconduct by a party to the trial, or a juror, or some outside actor such as an audience member can sometimes be so disruptive as to warrant the declaration of a mistrial. In the event that a juror is disqualified, and if no replacement can be found and the litigants can't agree to proceed with the remaining jurors, then an impasse is reached and mistrial may be declared. Sometimes, in the middle of a trial, the court may determine that it has no jurisdiction over the case, in which case a mistrial is declared. If it is discovered that members of the jury have discussed the case in violation of court instructions, or that a sequestered jury has had access to biased media coverage of court proceedings, a mistrial may be declared. And the sudden death of an attorney or other party to the trial can also be cause to declare a mistrial.