Now Playing is one those things that if you close it, it’s closed. You’ll have to use the dock switcher to get it back. Was very annoying in the betas but looks like it made its way into production.
Double click the Digital Crown and you get back to the last used app. So, if you've just dismissed the Now Playing screen, double click the crown and you are back.
Alternatively...
Press the side button and you get a carousel of apps, like this:
By default, they are listed in the order they were most recently used.
However in the Watch app on your iPhone, under Dock, you can change the order manually.
Personally, I have the Now Playing app as the first on on the list. That way, if I dismiss the Now Playing screen, I can quickly get back to it, even if it wasn't the last app I used.
I haven't explained this properly.
Close 'now playing' by swiping it.
Then press the large button to bring up the carousel.
'Now playing' is not there, nor is it there in the list of apps.
I just found IF you click the next track on your phone (don't forget, i'm using spotify) it reappears. It's like the music app is triggering now playing to appear again.
EdIT:
@jhfenton and @tech3475 Thanks! If it helps, this ONLY appears in the middle complication. I didn't see it either side