"Put on sackcloth, O priests, and mourn; wail, you who minister before the altar. Come, spend the night in sackcloth, you who minister before my God; for the grain offerings and drink offerings are withheld from the house of your God." NIV translation
Joel calls the priests to repent as sackcloth was a symbol of repentance and mourning. He started with the call to the priests because they were the leaders and they should lead by example. They are to mourn because God has kept them from presenting offerings and, without those offerings, the people could not have a right relationship with God. Just like in Joel's time, God is calling those that lead His people to lead by example.
"Declare a holy fast; call a sacred assembly. Summon the elders and all who live in the land to the house of the Lord your God, and cry out to the Lord." NIV translation
Joel then tells the priests to declare a holy fast and to hold a sacred assembly. A sacred assembly was a day where work was not allowed and all of the people were to meet together at the temple. This was a call for all of the people to go without food or drink and for them to get together to pray.
"Alas for that day! For the day of the Lord is near; it will come like destruction from the Almighty." NIV translation
Joel tells the people that even as bad as the locusts were that is not as bad as it gets. Worse is yet to come for those who do not repent as on the day of the Lord the destruction will be from El Shaddai. The name Almighty (El Shaddai) refers to the fact that God created everything and owns everything.
"Has not the food been cut off before our very eyes - joy and gladness from the house of our God? The seeds are shriveled beneath the clods. The storehouses are in ruins, the granaries have been broken down, for the grain has dried up." NIV translation
Joel begins to survey the damage once again and he starts with the crops of the field. Man plants the seed but only God can make it grow and so man's efforts here lie "shriveled beneath the clods". Not only were the people without food but they also did not have the offerings that were required for the temple.