Official English text from the Liturgy of the Hours:
O Key of David, O royal Power of Israel controlling at your will the gate of heaven: come, break down the prison walls of death for those who dwell in darkness and the shadow of death; and lead your captive people into freedom.
Official Latin text:
O clavis David, et sceptrum domus Israel: qui aperis, et nemo claudit; claudis, et nemo aperit: veni, et educ vinctum de domo carceris, sedentem in tenebris.
More literal English translation:
O Key of David, and scepter of the house of Israel, who opens and no man shuts, who shuts and no man opens: come, and lead forth the captive who sits in the shadows from his prison.