Q: I can't see image only sound on a video on the web.

As you probably guessed this is a complicated question.

Here are a few information gathering steps.

1. Is there something about the udemy Web site? Try a web site that should work e.g. https://www.apple.com/watch/films/
2. Find out and tell us the name and version of the browser you are using to watch udemy videos. The version information should be in the browser's toolbar e.g. Safari -> About Safari. Try a different browser.
3. I looked at one preview at udemy which played without a Flash plugin. Would you try the preview for Mac Essentials at udemy? Does udemy have any support/help? Maybe this is a familiar problem for them.
4. Make a new user on your computer and see if the problem persists with a fresh start, no old preferences or settings. Even logging in as Guest might be a way to test this.
5. What happens with non-streaming video? Do you have any movies or TV shows in iTunes that you could watch and see if the video appears. Or some other video source already on your computer?

If video isn't working in any form, and it previously worked on your computer, then I think it's time for a trip back to the person who put in the new ssd. Something must have gone wrong with the re-install.

Please let us know what you have learned and done.

dick glendon