If I'm understanding current rumors right, Luke Skywalker disappeared off the face of the galaxy almost as soon as the victory party at Endor ended. He'd decided (Yoda's "pass on what you have learned" be damned) that the Force was simply too dangerous for anyone to wield, that the temptation to go dark and start killing people was too strong. The only way the Force could truly be balanced was to take it off the table - no more Jedi, no more Sith. And eventually, hopefully, no more war.
So the Star Wars universe we see at the start of Episode VII is one where the Force itself is a fast-fading memory. No one - not even Leia - knows how to sense it, let alone use it.
No one, apparently, except for the cyborg now in charge of the Galactic Empire that nobody knew about back then. His injuries no doubt limit his power significantly (as it did Vader), but he's still got enough of the Force in him to be a threat, and now he's evidently manipulating Daisy Ridley's character (and eventually, Han and Chewbacca) from behind the scenes to lure Luke out of hiding. And once discovered, Luke realizes he's got no choice - either he trains Daisy as a Jedi, or he gives in to this cyborg (or his own inner darkness) and lets the Dark Side rule forever.