I had an issue the other day where a user reported that all of the documents had ‘disappeared’ from their respective document sets in SharePoint 2010.
Sure enough, when I opened the document sets, no documents were listed. If I created a view which didn’t display folders, I saw that the individual documents were still there.
After some analysis, I discovered that the site owner had changed the Welcome Page for the document set content type. When I changed it back to what it should be, the documents were then visible again.