This happened to me once as well. I left it in the carboy and continued to use the wine whip with two of the three "blades" on the other wines I was making. Soon the wine needed to be racked, so I was able to retrieve the lost blade. It was easy to reattach with an allen wrench. So, as long as you sanitized the wine whip (which I assume you did), there is no harm in leaving it in.