If it is denied and the jury acquits, there is no retrial. Not sure if there is a procedure for an emergency appeal prior to giving the case to the jury. For most situations, there is no appeal from a court ruling during trial; otherwise trials would get endlessly interrupted and appellate courts would see a huge increase in work load. So generally, you have to appeal after trial. There are a few exceptions, but my guess is that denial of a mistrial motion is not one of them.
Thanks