If you have multiple versions of ADT installed on your computer, you may want to be careful about opening content browser library files [CBL extension] in a version of ADT other than the one in which it was created. Let me state that I do not know for certain that caused the problem I had earlier today - perhaps some other form of operator error is to blame.
I was in the middle of trying to figure out where project-based palette groups were stored and ran across the folder where the content browser library files are stored on my computer. The files for both 2005 and 2006 are in the same folder, but have different names. At some point, I had the 2006 library - ContentBrowserLibrary47.cbl - open [by what means, I do not recall, so I am not certain that I opened it with the AecCB47.exe application] and decided to open a different library. When I tried to reopen the 2006 library file, it was empty - not a single catalog - and the title had changed from "David Koch's Content Browser Library" to "contentbrowserlibrary47". After getting over the initial shock, and a few vain attempts at reopening the file in the hope that is would "magically" restore itself, I set about chasing down the location of all of the catalogs that were open in it. My custom catalog was easy, it was remembering what the out-of-the-box catalogs were and where they were located that was hard. I may not have even got all of them, but I think I got most of them. I did decide not to open "MyCatalog", as I found that if I keep my custom files in that catalog, and then install a newer version of ADT, it overwrites the "MyCatalog" file.
One thing I did discover is that if you navigate to a CBL file in Windows Navigator, right click and choose Open With, the Content Browser application is listed only as "AecCB EXE" - there is NO indication of what version of AecCB will be used. My suspicion is that I inadvertently opened my 2006 library with another version, and that may have lead to the file getting "cleared". So if you have multiple versions of ADT installed on your computer, I would advise being careful when changing libraries within Content Browser, and only open libraries that were created with the version of Content Browser that is currently open. I would avoid double clicking on a CBL file. If you do not want to open ADT first, then open the desired version of the Content Browser by double clicking on the EXE file or using the Start Menu, then opening the desired library file [if it is not the current default file].
I did make copies of the CBL files, adding "_backup" to the file name, in case I manage to make a similar mistake in the future. The usefulness of the backup will depend, of course, on whether I remember to make a new backup if I ever add or delete catalogs from the library in the future.
It would appear that the problems I was having can be ascribed to "operator error". It is still a good idea to regularly back up your CBL file.