Premiere Elements 11, what computer operating system?
A Premiere Elements user ELM312 ran into and personally solved the same type of issue that you are reporting.
As per ELM312 solution, please go back to Edit area and check your Timeline content to assure that there are no media disconnects.
Please refer to
and let us know the outcome.
Problems with the same symptoms can (often do) have different causes. So, ELM312 solution may not be yours. But, worth looking at.
We will be watching for your results.
Correct answer! There was a media disconnect (one of my still photos had disappeared). I replaced the photo, and all was back to normal.
Great news. Great job in follow through. Much appreciated those words "...all was back to normal".