Does this happen frequently?
I assume your are simply dragging the sheets to their new position, right?
I guess you are sure that the subset was not inadvertently dragged into another subset?
Have you expanded all subsets to make sure it's actually gone and not just moved?
(Sorry for some obvious questions, but we need to start somewhere.)
There is a backup saved of the .dst file. Look in the same folder where the .dst file is for a similarly named file. You can rename the backup extension to .dst to restore the previous sheet set. You may have to turn on file extensions in the view settings of your Windows Explorer. (assuming Windows).