This won’t mean that you will be grounded forever. If you are denied or your results are deferred to the FAA for review, you can appeal the decision with the FAA. This should only be done once an outright decision has been made after a referral.
If you are denied because of substance abuse and you can prove that you have been to rehab and have been sober for at least 24 months, you can appeal.
If you failed on health problems, you can usually obtain a special issuance medical certificate after the deferment process with the FAA is completed. You may need to change some medications to FAA-approved ones on occasions and wait until you are symptom-free over a period of time.
Most of the time, the FAA will allow you to fly if you are relatively healthy and fit. If you believe the results are wrong, challenge the decision.