What happens when the seventh Trumpet sounds?
When the seventh trumpet sounds, heaven declares that the kingdoms of this world have become the kingdoms of the Lord and of His Christ, and He begins His eternal reign, followed by worship, the pouring out of God's wrath, and dramatic signs in heaven.
When the seventh angel sounds his trumpet, a great announcement rings out in heaven: the kingdoms of this world have become the kingdoms of our Lord and of His Christ, and He shall reign forever and ever. Loud voices proclaim Christ's eternal reign, and the twenty four elders fall down and worship God, giving thanks that He has taken His great power and begun to reign.
Along with this declaration comes the announcement that God's wrath is coming. This is described as the time for judging the dead, for rewarding the righteous, and for destroying those who destroy the earth.
At this same moment, the temple of God in heaven is opened, and the ark of His testament becomes visible within it. This is followed by lightning, thunderings, an earthquake, and great hail, marking the seriousness of what is unfolding.
So the seventh trumpet is not simply another judgment among the trumpets but a turning point: it announces that the kingdom has come, that Christ's reign has begun, and that the final acts of God's wrath and judgment on the earth are set in motion.
The scriptures
And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever. And the four and twenty elders, which sat before God on their seats, fell upon their faces, and worshipped God, Saying, We give thee thanks, O LORD God Almighty, which art, and wast, and art to come; because thou hast taken to thee thy great power, and hast reigned. And the nations were angry, and thy wrath is come, and the time of the dead, that they should be judged, and that thou shouldest give reward unto thy servants the prophets, and to the saints, and them that fear thy name, small and great; and shouldest destroy them which destroy the earth. And the temple of God was opened in heaven, and there was seen in his temple the ark of his testament: and there were lightnings, and voices, and thunderings, and an earthquake, and great hail.
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.