The Good, the Bad and the Cursed - S3-E14
Corrected entry: In "All Halliwell's Eve" the charmed ones go back to the past and don't have their powers because they wern't born yet. But when Cole and Prue go back to the Wild West, Prue keeps her powers even though she hadn't been born yet.
Correction: They don't actually go to the past. They go to an alternate realm/plane where the inhabitants are forced to relive the same time and events over and over because they keep making the same 'wrong' choices.