He was deposed from his kingly throne - Became insane; and the reins of government were taken out of his hands.
Matthew Henry
Concise Bible Commentary
Daniel reads Belshazzar's doom. He had not taken warning by the judgments upon Nebuchadnezzar. And he had insulted God. Sinners are pleased with gods that neither see, nor hear, nor know; but they will be judged by One to whom all things are open. Daniel reads the sentence written on the wall. All this may well be applied to the doom of every sinner. At death, the sinner's days are numbered and finished; after death is the judgment, when he will be weighed in the balance, and found wanting; and after judgment the sinner will be cut asunder, and given as a prey to the devil and his angels. While these things were passing in the palace, it is considered that the army of Cyrus entered the city; and when Belshazzar was slain, a general submission followed. Soon will every impenitent sinner find the writing of God's word brought to pass upon him, whether he is weighed in the balance of the law as a self-righteous Pharisee, or in that of the gospel as a painted hypocrite.