Hi Olivia,
I hope this reply isn't coming too late! I generally see this when there's a slow upload speed. Here's what I recommend:
If you're still on that screen, try right-clicking the video pane and selecting to "save as" to download the video you've already made. If you're no longer on this screen, you'll need to remake your video.
Next, if the assignment permits, I recommend using some other media storage service to upload your video. Does your school use Kaltura? Google Drive? Box? Something like that. These are much better at media uploads and media rendering than Canvas.
Finally, you can share a link from whatever service you use or simply embed your video into your assignment submission page. Make sure to give your teacher a heads-up, too, just in case he/she isn't expecting a video to be submitted in this way.
I hope this helps!
Nathan, Ogden, UT