John Gill Commentary Psalms 104:29

"Thou hidest thy face, they are troubled; Thou takest away their breath, they die, And return to their dust." — Psalms 104:29 (ASV)

You hide your face, they are troubled
God may be said to hide his face from the creatures when he withholds their food from them, when there is a scarcity of provisions, a famine in the land; when there is no pasture for them to feed on, nor brooks of water to drink of; then are they troubled or perplexed, as in (Joel 1:18Joel 1:20) and do not know what to do, nor where to go for help, but faint, and sink, and die. So in a spiritual sense when God hides his face from his people, removes his Shechinah, or divine Majesty and Presence, as the Targum here; and withdraws the influences of his grace and Spirit; or when they have no food for their souls, or what they have is not blessed, then are they troubled, (Psalms 30:7) .

You take away their breath, they die, and return to their dust ;
their original dust, from which they sprung, as man himself does; the breath of all is from the Lord; he gives it to his creatures, and when he pleases he takes it away; and when he does, they die and become dust again.