Answer:
In the Middle Ages, the concept of classes was very strong, but the term did not yet exist.
There was a middle class, but it did not have a name, beyond being called commoners. The problem with this is that the middle class, as we know it, is the class between the labor class and the wealthy. In the middle ages, commoners included everyone who was not a member of the royalty or peerage. And so a commoner could also be a member of the nobility, but just not a peer, just as today, the grandchildren of Queen Elizabeth II are technically commoners. But a serf, who was definitely not middle class, was also a commoner.
So the middle class consisted of commoners, but only those commoners who were not nobility and not simple laborers. There were always members of this group, from the very start of the Middle Ages; they just did not have a specific name associated with them that I can find anywhere until fairly recently.
Nevertheless, the rise of the middle class was one of the most important trends of the Middle Ages, and one of the most important causes of the Renaissance.
There are links below to articles on commoners.