If the jury cannot reach an unanimous verdict, the judge can declare a mistrial. A mistrial means that the trial concluded without a decision for guilt or innocence.
A mistrial can also occur during the trial if one side violates certain rules of evidence. For example, if a defendant has a past criminal conviction, the prosecutor usually cannot mention it during trial. If the prosecutor improperly brings up the criminal conviction, the judge will probably declare a mistrial.
When a trial ends in a mistrial, the state may usually try the defendant again. However, sometimes a mistrial suggests to the state that they have a weak case, and they will dismiss the charges.