If you've done the other pieces and sections well, then you will pass.
When I have my piano Grade 8 exam, I suddenly stopped in the middle of a piece. I actually didn't know where I had played up to. All I could do was ask the examiner again (shamefully) if I could start the piece all over again. The examiner was very nice and allowed me. And I passed Grade 8. (though I could have been so much better in terms of marks if I hadn't slipped)