Failing the annual MOT assessment is common.
If you do happen to fail your exam, you will need to retake it. Each exam purchased includes 1 free resit.
There is no limit on the amount of attempts you can have, although it may impact on your individual RAG score with the DVSA if you are logging a number of failure results.
The DVSA do advise that in the event of a failure, the tester should wait 7 days before attempting a retake. During this time, the tester should go back over their training and specifically any topics that resulted in the exam failure.