iMovie video won't play; Sound will, thumbnails show, no video

So I recently got some old cassettes transferred to CD, and then converted to MP4 via Handbrake, and was finally able to import the video into iMovie after some very strenuous forum-searching and keyboard-smashing. (I run an older iMovie model, '11 or so). I imported the video, and the thumbnail shows up, the audio plays fine, but there will be no image (no video). Like the thumbnail, audio and everything is there and plays, but the actual FOOTAGE doesn't visually play. I have tried deleting the cache.pst, and restarting my computer but to no success. I am trying to piece together a video by Christmas and any help would be extremely appreciated.