O.K. To rule out an issue that may be confined to just one project, create a new project and transfer the old project's media into the new project. If you want to preserve your edits, do an Edit/Select All, Edit/Copy, of the media in the old project and then do Edit/Paste of the media into the timeline of the new project. Now see if the video will save when you export.
If still no luck, open one of the clips in QuickTimePlayer and do a Window/Show Movie Inspector. What is the exact format, audio and video, that is shown for the clip?
Whatever the format, try converting the troublesome clip to H.264, Mp4/AAC with the free download, Handbrake. Do that even if the codec already is indicated to be Mp4/AAC. Re-rendering often cures the issue.
You can get Handbrake here:
https://handbrake.fr/
A simple way to do it is to open Handbrake and do a File/Open Source. Navigate to your video and choose it as the source from the resulting screen. Then do File/Start Encoding. Wait a couple of minutes for the conversion to complete. Then save and import the converted clip into iMovie. Now try exporting the clip and see if you get video.
-- Rich