I'm not sure how I did it, but as I moved the mouse around the interface to apply an effect, I clicked inadvertently somewhere.
The net result is that I lost the timeline panel. When I went to the menu to attempt to "restore" it, it placed a timeline panel in the lower part of the page that does not look correct, does not contain the clips I had placed and saved in the timeline, and doesn't allow new clips of any kind to be moved in.
When I try to open a previous project, I get the same interface, so I'm hoping it's something that can be corrected at that level, and that it has not affected the project content per se.
Thanks in advance for any help...