I think you are referring to the (re)discovery of the Torah scroll by Chilkiyahu the High Priest in the time of Yoshiyahu (Josiah?) (mentioned in Kings II chap. 22, and Chronicles II chap. 34), in the course of renovations to the Temple. (If I'm mistaken, please cite a source).
You are quite correct that there were many copies of the Torah. This particular scroll was unique because it was written by Moses' hand (see Chronicles) -- the one in which Moses recorded the Torah as it was dictated to him by Hashem.
Yoshiyau tore his clothes in fear on hearing about it, because it was opened to the section in Deuteronomy of the punishments, particularly 28:36 - "Hashem will lead you and the king which you will set over you to a nation which neither you or your forefathers know" (free translation) (see Yoma 52b).