A fail is not the end of the world. You get a refusal certificate listing the faults, split into dangerous, major and minor. Dangerous items mean the car should not be driven away at all.
If your old certificate is still in date and none of the faults are dangerous, you can normally drive the car until it runs out. Otherwise it can only go to a booked repair or retest.
Bring the car back within 10 working days and the retest is free — the rules and booking details are on our MOT testing page.
We always quote for the repair work first, so nothing gets done without your say so.