I've used Zotero for a year. I moved to Zotero because I had major issues with Endnote and Mendeley, particularly with stuff disappearing. I just recently had same issue with Zotero. I have several layers/levels of collections in one library. I recently noticed where one second-level collection that had probably two levels within it went blank. The collection folder is still there, just none of the sub-collection or references. Luckily I had a fairly recent backup in another library (I learned my lesson from past) I could use to rebuild it. But, is there any way of figuring out what happened? I am pretty sure it wasn't just user error (accident), since about a half dozen sub-collections disappeared, rather than just one.