Are Hades/Sheol and Gehenna the same place? And if they are two different places, what happened to Hades after Christ resurrected?
Sheol/Hades, and Gahenna are all words for hell used in the Bible but they mean very different things.
Sheol is the Hebrew word for the realm of the dead used in the Old Testament. It is sometimes translated as “the netherworld” and it is the resting place of the good (Genesis 37:35) and of the evil (Numbers 16:30).
When the OT was translated from Hebrew to Greek around 200BC the word Hades was used. Today it is still common to see Hades used over Sheol in English translations of the OT because in the early Church (and really for most of Christian history) the Greek OT was the preferred version. Of the 350 OT quotes in the NT (mostly made by Jesus and Paul) around 300 of them quoted from the Greek OT and around 50 quoted from the Hebrew OT.
Sheol/Hades is the same place as the “bosom of Abraham” where both Lazarus and the rich man found themselves after death (Luke 16:19-31). By the Middle Ages it was most commonly referred to as Limbus patrum or the Limbo of the Fathers. It is where the righteous and the unrighteous alike were gathered in death until the Resurrection of Christ. Until Christ redeemed mankind heaven was not reachable. At his death Jesus entered Hades and took the righteous dead with him to heaven. This is known as the “Harrowing of Hell” and is what is meant by “He descended into hell” in the Apostle’s Creed and is likely what is referred to in 1 Peter 3:19 and 4:6.
Gehenna is the Greek word used in the New Testament that corresponds to what we typically think of as hell, sometimes called perdition. It is the eternal punishment (Matthew 25:46; Mark 9:42-48; Revelation 14:11, 20:10; etc.) of the damned.
The book of Revelation shows Hades (Sheol, the bosom of Abraham, the Limbo of the Fathers) itself being cast into hell (Gehenna) at the Final Judgement. (Revelation 20:11-15)