The lost hotel is said to be a hotel that was once quite famous.
But for some reason it has been left in ruins now, as you can see from the exterior. It's quite possible some major calamity like earthquake, storm or even a legendary Pokemon (quite possible Yveltal, being the death Pokemon that once did do something like that) happened to it.
However all this is speculation, nobody will know what the history behind the hotel is unless Nintendo or Gamefreak make it official.
Since the lane on which Lost Hotel lies also homes quite a few Hex Maniacs though, it's quite possible an ill omen or somthing occurred, as they usually tend to gather in ominous places.
Now it's nothing more than a hangout spot for the best skater in all of Kalos and some other Punks, all wannabe skaters themselves.
Hope I helped!
