Exported .mp4 won’t play (both audio and video)

export in finder has thumbnail; can scrub to see image

-the exported mp4 does play in premiere totally fine when imported....
-I also tried exporting the .mp4 into media encoder, and that export didn’t play either
-all settings are standard h264, etc, and used on other projects which export working mp4 no problem.
-I’ve turned off Mercury playback engine to use ‘software only’….
-I’ve cleared the media cache….
-I’ve cleared all render files and re-rendered….
-I’ve nested the timeline….
-I’ve started a new project with only the Final Cut sequence…..

Guh. And the film is finished. And I am stuck in FML-land. Please send rescue squad.