If a jury cannot reach a verdict, a judge can discharge that jury. In this case, Double Jeopardy Clause allows for the defendant to be re-tried.
The most recent pronouncement is from the United States Supreme Court in Renico v. Lett, 130 S.C.t 1855 (2010). In Renico, the United States Supreme Court provided guidance for reviewing a decision of a trial court to declare a mistrial based on a hung jury:
A State may try the same defendant twice if the jury is deadlocked. However, if the trial judge's basis for a mistrial ruling is unrelated to the problem the trial is addressing, close appellate scrutiny is warranted.
The U.S. Supreme Court has also refused to apply a rigid formula for deciding in what cases a deadlocked jury warrants a mistrial, because there are too many case-by-case variables. If a trial judge is acting irrationally, that would be an example of where declaration of a mistrial could not be condoned. The Court also says that a trial judge is not required to make clear findings of manifest necessity nor to 'articulate on the record all the factors which informed the deliberate exercise of his discretion.'
And the United States Supreme Court has never required a trial judge, before declaring a mistrial based on jury deadlock, to force the jury to deliberate for a minimum period of time, to question thejurors individually, to consult with (or obtain the consent of) either the prosecutor or defense counsel, to issue a supplemental jury instruction, or to consider any other means of breaking the impasse.