Which collection mode allows drag-reordering?

I have 400 images in a Collection under a Collection Set heading and I was happily dragging them into the order I wanted - all originals are in the same folder. But then a message brielfy popped up on the screen that I could not read - something about how to reorder files, after which, I can no longer drag and reordered the files I had just been doing so with. Not sure if I hit some key errantly and switched modes of some sort - still looking at the Collection in the left column - but now I can't reordered the files. Which mode should allow this? Thanks for any info.