You must have switched to "Hardware" or "Software" decoding mode from the default "QuickTime". Then, if you don't enable mirroring (and, with AirPlay & video playback, you NEVER should), there will only be audio output.
I've written a mini-tutorial on switching between these decoding modes (also) in my latest nPlayer review here at MR:
https://forums.macrumors.com/threads/1652736/ (see the first three screenshots in Section 2).
All this assumes you're trying to play back native iOS videos - that is, mov / mp4 / m4v files.