Old Testament saints (people of faith in the one true and eternal God Jehovah) who died prior to Christ Jesus' ascension to heaven were held captive in a place located in the heart of the earth known as Sheol (Old Testament) or Hades (New Testament). 

Hades, at that time, consisted of two areas. One area was known as Abraham's Bosom, or Paradise, and the other area, was, and still is, the area of the unrighteous dead (the unsaved). (Luke 16:19-31)

It was to Abraham's Bosom (Paradise) to which Jesus descended after His crucifixion and prior to His resurrection. He went there to proclaim victory over sin and death on the cross of Calvary.

When He ascended to heaven forty days after His resurrection, Christ Jesus took all who were held captive in Abraham’s Bosom (Paradise) to heaven with Him. (Ephesians 4:8)  Paradise is now in heaven. (2 Corinthians 12:2-4)

The unsaved, however, are still in Hades awaiting their resurrection and judgment of sin. (Revelation 20:11-15)