Maybe you had not enough practice, maybe the instrument is unfamiliar, maybe the piece is rather new to you.
Do you have a chance here? Can you still perform at a high level? Or is it all over?
The thing is, you have to remember, that you are facing yourself in such situation - your fears and insecurities, your passions and dreams. During the performance it all comes back to you.
But you can learn to control your feelings, you can focus your mind, your can be exclusively concentrated on your breath and on what's happening right here and right now, e.g. the current measure.
Yes, you might be unprepared but you can be ready.
Then you have a chance.
It's all mental.