You create the grain, you bring forth the barley, assuring perpetuity to the temples. if you cease your toil and your work, then all that exists is in anguish. to whom (or what) is this hymn most likely addressed?

This hymn is most likely addressed to the river Nile.

Explanation:

In the "Hymn for the Nile", the ancient Egyptian people sung about the importance of the river in their everyday lives. This tune that was created and sung by the ancient Egyptians gave obeisance to the river for the sustenance of their lives.

The lines "You create the grain, you bring forth the barley, Assuring perpetuity to the temples. If you cease your toil and your work, Then all that exists is in anguish" gives credit as well as asking for it's continued assistance. The personification of the river by the pronoun "you" gave it an entity that is living and powerful. "If you cease your toil" shows the efforts of the river to provide water for the place and not some bad disaster. Without the flood, "all that exists is in anguish", thereby bearing the image of some powerful being responsible for their survival.

Though the river floods regularly, it is this same flooding that enables the vegetation and livestock to sustain their lives. Even called as the 'gift of the Nile', Egypt has always relied on the Nile river. So, the floods that may seem like a misfortune for some people is in fact more of a blessing for the people living near it. The desert of the Sahara and it's surroundings owes their existence on the Nile river.