The sealing of the Bajoran wormhole at the end of 2374, when a Pah-wraith released by Gul Dukat cut the Prophets off from Bajor, until Benjamin Sisko reopened it about three months later.
The closing of the Bajoran wormhole happens in the Deep Space Nine episode Tears of the Prophets. Dukat persuades the Dominion that he can remove the Federation's advantage at the wormhole, and he frees a Pah-wraith from a Bajoran artefact. Possessed by it, he enters the station's shrine, kills Jadzia Dax and lets the spirit pass into an Orb. The wormhole collapses and every Orb on Bajor goes dark.
Bajorans are left without their gods. The cult of the Pah-wraiths grows, and Sisko, cut off from the Prophets and mourning Jadzia, goes home to New Orleans.
In Image in the Sand and Shadows and Symbols a vision sends him to the desert planet Tyree with his father, his son and Ezri Dax. He digs up the Orb of the Emissary and opens it, the Pah-wraith is thrown out, and the wormhole reappears. He also learns that a Prophet inhabited his mother Sarah to bring about his birth.