In regards to Luke's previous lightsaber, that lightsaber wasn't exactly his, as it belonged to his father, Anakin Skywalker. Luke built his own lightsaber shortly before the events of Return of the Jedi. Luke did this for two reasons, 1 - to replace his previous lost saber of course, and 2 - if he's able to build his own lightsaber, then that shows how far his training as a Jedi has come along. But also, the change of lightsaber colour (blue to green) was due to film reasons. In Return of the Jedi, Luke was originally supposed to have a blue lightsaber, but they found that the lightsaber blade was too difficult to see during the sail barge scene. So they changed it to green to help make it stand out. They also wanted to surprise the audience with "Oh he's got a new lightsaber!" As to who retrieved his lightsaber or why, no idea, that's an interesting question that will hopefully be revealed in the next film.