In Star Wars: The Empire Strikes Back we see Darth Vader cut Luke's right hand that is holding the lightsaber, and both fall down in the abyss.
In fact, in Star Wars: Return of the Jedi the lightsaber that Luke uses is a his own creation (we can see Darth Vader congratulate him for the job when he takes the lightsaber on Endor).
So, how can Maz have the lightsaber in Star Wars: The Force Awakens?