Look in your results envelope to see if it tells you what to do about this.
Contact the school to find out your marks and so whether you were close to the next grade up or not - if you are close to the 3/4 boundary do not reisk a review of marking.
Talk to them about whether you should go for a review.
However, TSR is full of threads like this and, for the majority of students, their grade will be correct and they just failed to do well enough on the day. Sadly, life is like that.