Is there a name for knowing something is gone but not remembering what it was?
Sometimes I'll notice that a building that I used to drive by all the time is gone (leveled to the ground). I know it's gone, but I can't remember what it looked like or what was in it. I just know it's gone.
There's probably some really specific word in German for this, but I can't think of a single word in English that would fully describe what you're talking about.
That totally happened to me the other day when I was visiting the town I grew up in. It's only a little over an hour away by car, but I hadn't walked around the downtown in years, and it's so different that I couldn't even remember what buildings used to be when they were replaced sometimes.
It's not a perfect match to what you describe but it's defined as "the phenomenon of experiencing a situation that one recognizes in some fashion, but that nonetheless seems novel and unfamiliar."
In your case, you recognize there was a building, and recognize it should be familiar, but it still feels unfamiliar to you nevertheless.