"Grain offerings and drink offerings are cut off from the house of the Lord. The priests are in mourning, those who minister before the Lord." NIV translation
Because of the locusts, there is no grain or grapevines left and therefore nothing to offer God at the temple. The priests are in mourning because they see that, without the offerings, the people are stuck with their sin. We can rejoice in the fact that Jesus was the perfect offering and so we are not required to present these offerings.
"The fields are ruined, the ground is dried up, the grain is destroyed, the new wine is dried up, the oil fails." NIV translation
The grains are gone as well as the grapes and the olives for oil. The ground even mourns ("is dried up") as it suffers the effects from the sin of the people.
"Despair, you farmers, wail, you vine growers; grieve for the wheat and the barley, because the harvest of the field is destroyed." NIV translation
The locusts affected everyone and they are called to mourn. This is usually how it is with many people as they normally are focused more on earthly material things than on the things of God. Therefore, when it is necessary to get their attention, the things that they are focused on are struck such as here with the farmers.
"The vine is dried up and the fig tree is withered; the pomegranate, the palm, and the apple tree - all the trees of the field - are dried up. Surely the joy of mankind is withered away." NIV translation
Joel continues by explaining that all of the fruit trees have been destroyed and no longer bear any fruit to be harvested. They forgot that God was the one that caused the trees to bear fruit and now their hope is gone. If we remember that God is our provider and we trust in him, then we will never lose our hope or joy even in the difficult times.