First, make sure you have copied the entire embed code correctly, then try adding the video again. If you are still having issues, your embed code may not be formatted correctly. Here is an example of a correctly formatted embed code from YouTube:
If you are embedding a video from YouTube or Vimeo, your code should look like this, with the exception of different numbers for width and height, and a different url for src.
If you are having issues embedding a video that uses "embed" or "object" tags rather than "iframe", then please contact the Help Desk directly.