My mp4 videos only play sound no picture

I have been exporting videos out of final cut pro for use on Aplle devices (mp4)

When these have exported there is sound but no video picture, just a black screen.

I have then gone into my files to try and play other mp4's and videos that used to play fine, now have the same problem?

Only a week ago I uploaded a partcular video to YouTube and posted some on FB so I know they exported okay a week ago, but somethings changed??