And learn you completely left out the original spark, so it comes back.
Losing interest in a story makes it hard to write, but it's one charm of finishing it: the story is done, the idea is pinned down the page, and so it's out of your head.
But ideas can undergone all sorts of modifications on the way to the page. Sometimes, the inspiration goes with them. Sometimes it doesn't.
It's most annoying when it's an idea ripped off from another story. Go back and file off the serial numbers again, chose a genre, do some more introductions -- and this time you have to ensure it's not only different from the original, but from the first attempt.
Unless, of course, the first attempt died. Still, you know you need something New and Different to get it again. Unless of course it's possible that you just need to get the first one going again with some modifications to how it goes. . . .