Where did he find the Torah?
Rashi, Radak (both in Melachim II, 22:8): It was under a row of rocks. It was hidden when Achaz 1 burned the Torah.
Radak (Melachim II, 22:8): It was in its place, next to the Aron, like it says here "when they were taking the money out of Beis Hashem, Chilkiyah found the Sefer Torah."
Radak: This is difficult. Refer to 34:14:3:1. Alei Tamar (Shabbos 16:1) ? Rashi and Radak cite a Midrash that we do not have. Sanhedrin 103b says that Achaz sealed (stopped study of) Torah, and Amon burned the Torah. Ya'avetz (on Seder Olam 24) fixes the text of Rashi to say Amon. Targum Rav Yosef on verse 15 says that the Sefer was found Ganun (or Ganuz, i.e. hidden).
Why does it say "b'Yad Moshe"?
'Rashi': It was Sefer Devarim 1 .
Malbim: It was the special Sefer, that Moshe wrote . This is why it says (verse 15) "Sefer ha'Torah" with the prefix Hei. It was very valuable, and it was hidden in the days of Menasheh and Amon.
One may not write a partial Sefer, but one may write an individual Chumash (Rambam Hilchos Sefer Torah 7:14). The other Chumashim, Hashem put the words in Moshe's throat. Sefer Devarim, Moshe himself said it (Daf Al ha'Daf Megilah 31b citing Tif'eres Yisrael 43 and the Vilna Gaon, but some disagree). (PF)
Rashi (Melachim II, 22:8) writes that the Sefer was hidden when Achaz burned the Torah. After Achaz, Chizkiyah spread Torah (Sanhedrin 94b). Why did he not take the hidden Sefer? And when Chizkiyah died, he left many Sifrei Torah!
Radak (Melachim II, 22:8): Menasheh was king for 55 years. He made Torah forgotten; people turned to idolatry and Nochri rituals. Now, Yoshiyah commanded to look for every place that needed fixing, and to take out the money brought to Beis Hashem. They found the Sefer Torah next to the Aron (refer to 34:14:1:2). They read it, and found that the Torah forbids all the bad things that Yisrael were doing.
Perhaps indeed, there were Sifrei Torah. They were alarmed to find the Sefer next to the Aron rolled in the middle; it should be rolled to its beginning (Bava Basra 14b). They found that it was open to the curse that the king and nation will be exiled. This aroused them to realize that they had sinned, and to repent. (PF; perhaps this is the intent of Malbim Melachim II, 22:8.)
