It happens I hate to say. I sometimes go into a meet all amped up only to have the switch turn off for no reason.
Then the next time I see the same guy, it's all good. It used to get me upset, but my good regulars know how to change things up and turn it into a positive. That trait is a big part of what makes them providers to value.
I have one that is smoking hot gorgeous and its happened. He will then suggest we shower together and sometimes that gets things going, or it doesn't. But he always makes it Ok.
All I can really say is dont get overly stressed..its not the end of the world. Make the best of it.