Since the spacecraft in Pandorum has already fallen to the planet Tanis, why does the movie show the spacecraft floating into space in the start? There is no evidence that this is a flashback.
That event happens long before the events of the movie (I'm quoting from Wikipedia):
Eight years into the mission the ship receives a transmission from Earth in multiple languages: "You're all that's left of us. Good luck, God bless, and godspeed."
But the ending reveals:
However, what Gallo's hallucination said about the ship being lost in space was a red herring. As Nadia observes bioluminescent ocean life through the windows, and the computer displays that 923 years have elapsed since the mission launched - 800 of which the ship has spent stationary and underwater after arriving at Tanis and automatically landing itself in the ocean.