Thankfully, 'A' circuit shows have at least one on-site farrier for shoeing emergencies such as these. If you can find the shoe and bring it with you to the farrier, you should be back in action in no time. If you can't find the shoe, you will have to spend more time at the farrier, but you'll still have a chance of being able to return to your class before it finishes.
On-site farriers are not limited to the 'A' circuit. Some farriers do attend 'B' circuit and schooling shows but you cannot generally assume that there will be a farrier at those shows.
The prize list for an 'A' circuit show will usually list the name of the on-site farrier on the same page as the officials. To find out where the farrier is actually located on the show grounds, ask the show office. Sometimes they use a special tent, sometimes they use a regular tent stall and sometimes they use an aisle-way or grooming area in a permanent barn. To avoid wandering around the horse show for hours looking for him, it's best just to ask!
In addition to replacing lost shoes, the on-site farrier can also drill threaded holes into your horse's shoes for studs (it's best to have your regular farrier put stud holes in your horse's shoes in advance to avoid such emergency farrier visits). He can also put new shoes all around on your horse or do a reset, but beware that most farriers will charge a premium for regular farrier work done at a horse show. In some cases, having your horse's feet done at a show can give you the opportunity to use a particularly good farrier who does not travel to your area, so you must weigh the pros and cons.