Hi,
First, to cover the bases, make sure that you have not inadvertently selected the Audio Only option in the share box.
If that's not the problem, assuming that you are using iMovie 10.x.x try the following diagnostic procedures, if you haven't already:
1. Restart your computer and launch iMovie while holding down the Option and Command keys, and then deleting preferences from the box that appears.
2. If that doesn't work, try doing an Edit/Select All/Edit/Copy of your project, and then Edit/Paste the project's media into a newly created project.
3. If that doesn't work, try opening iMovie in a newly created user account. You can use your Mac's guest account for this purpose, or create a new user account from Apple/System Preferences/Users and Groups.
If this issue is not happening in other projects with other media, possibly your video media in this project is in a format that is not supported by iMovie. You may already have checked that. If not, try converting your clips to Mp4 using the free download Handbrake. Do this even if the clips already are Mp4.
If the issue continues to happen in other projects as well, you probably are looking at doing a reinstall of iMovie. Upgrade to current operating system (Sierra) and iMovie version (10.1.5).
-- Rich