I put 3 mp4 videos on my website. When I preview the webpage and play the videos, I get sound, but no video.

Background: I wasn't always having this problem. Today when I went to open the website to work on it, it said it was already open (which it wasn't) and gave me the option to load a copy. I opened the copy and saved it as a new file. Now the videos don't work right in that new file. I made a lot of big changes and do not want to have to revert to the old file.

Thanks in advance for any help!