clicking on thumbnail no longer starts slideshow

Up until recently, my galleries were working correctly. The thumbnails were displayed correctly, and when I clicked on one of the thumbnails, the full-size version of the image would open in the image browser thing, with Prev and Next links displayed below it. Clicking on those links would allow you to browse through the images in the gallery. But sometime in the last week, that stopped working. Now, when I click on one of the thumbnails, the full-size image appears in the browser window alone, with no border, no Prev/Next links, no caption, nothing.

I’ve looked through all of the options, and can’t see that anything has been changed, accidentally or otherwise. I have no idea what happened. Any ideas?