A Crisis of Confidence.
There are tips to prevent it.
There's even a movie about it.
I believe every performer has experienced this at some point in one's career.
What exactly do I mean?
Attack of Nerves.
Stage fright is categorized as a subset of glossophobia, or fear of public speaking
Symptoms of Stage fright: Dry mouth. Tight throat. Sweaty hands. Cold hands. Shaky hands. Nausea. Fast pulse. Shaky knees. Trembling lips. Any out-of-the-ordinary outward or inward feeling that occurs before, or during, the beginning of a performance.
Even *S*T*A*R*S* get stage fright.
The first step is realizing it is happening, understanding it is normal, and talking with other musicians about it. You may be able to learn something from their experiences that will help you.
Second, I cannot stress the importance of preparing fully for a performance. PRACTICE!! There is a difference between honest-to-goodness stage fright and lack of preparation. If you are prepared, drawing upon muscle memory or innate responses can serve as back-up and potentially get you through.
Third, if it happens (and it likely will), know that unless there is something medically wrong you will survive whatever symptoms plagued you. Learn the best way you can prevent it and deal with it.
It is unsettling...scary even.
It is embarrassing.
But, it is not the end of the world.
You will go on to sing or play another day.