Export is only audio :(

Hi! as the title says, i've been trying to export an hour long video on iMovie and it's exporting only the audio. says it's an mp4 file, but i open it with vlc and quicktime and it plays only audio. i've done it several times, each time taking about 6 hours, making sure i'm checking the audio and video option on export. i've tried exporting the video directly to my mac, then to an external hard drive, i've freed up to 30 gb of space on my mac, i've tried exporting it with the faster option and with the better quality option, i've tried exporting from the project page and from the main page where all my projects are. i've duplicated the project and tried exporting that one. nothing works. my computer and iMovie are up to date. my computer may be old (2017 mac air) but i've been able to successfully export videos before with no issue. please help!