If the worst happens you will need to take the driving test again. You must wait a minimum of ten working days before attempting the driving test again. You can, if you need, take the driving test an unlimited number of times before finally passing. Just be aware that your theory test pass certificate expires after two years from the date of issue!
If you were unhappy with the result and feel the driving test was not conducted fairly or properly, you may appeal. Ask your driving instructor for advice on this. In very rare cases a free retest may be given but the driving test examiners decision is final and will stand.