“exhausted.” This Hebrew word is unclear, but this seems to be the general meaning of the text, and most translators agree (cp. “fainted,” JPS, KJV; “weak,” CJB, NAB; “overcome,” NLT, NRSV). Exhaustion and weakness is also a natural human reaction to emotional shock and mental stress. To us today, Daniel’s vision was mostly of ancient events that are long past, but to Daniel, those events were coming upon the world and his people, the Jews, and as we now know from history, would have a great impact upon them. Besides, although the vision included successive empires, there was no portrayal of the restoration of the Jewish nation. There would be centuries of heartache before then.
“could not understand it.” This seems to be a natural understanding of Daniel’s situation, although some scholars think the Hebrew is more like, “there was no one to explain it.” But even so, that would result in Daniel’s not understanding the vision.