Another guy did the same thing (posted on reddit):
"So it was poorly lit and I was tired, I just put a strap thingy on the new joycons and well I did it the wrong direction and it sort of acted like it was going on, but then shot a bit past the edge and now I can't move it and it is stuck and I am very sad
That is all, everyone just be aware you CAN put them on the wrong way and they get stuck
EDIT: Okay that was terrifying, after inspecting the other joy con and playing with it a bit and looking at the mechanical part, I decided to go ahead and be brave and just try to slide it back from where it came while holding in the release button, it was scary, and sounded bad, but it did come off and looks to be intact on all connections. I know I'm way too sleep deprived and sick, I shouldn't have been fucking with it."