I would argue that it is somewhat accurate to call the Renaissance a rebirth, but that it is overstated to say that everything that happened in the Renaissance represented a huge break from the Middle Ages.
It is not as if the ideas of the Renaissance sprang up all of a sudden out of nowhere. Instead, they came out of things, like the expansion of trade and of intercultural contact, that were going on in the High Middle Ages. Therefore, it is not fully accurate to say that the Renaissance was a rebirth of a lost civilization.
However, the Renaissance was certainly a distinct period in history. The ideas of the Renaissance, like the idea of scientific inquiry, were different from the ideas that had been held in the Middle Ages. These ideas grew out of the Middle Ages, but they changed and became more pervasive in the Renaissance. This justifies saying that the Renaissance was a rebirth.