Oops...
Seems like you created a new library database to "D:\Films" with Library Manager.
Library Manager is a tool for handling the library databases. It is not for defining the media file locations. The import features & settings are in Tools > Import... (and also in Tools > Options > Library & Folders > Auto-Import Folders...)
Matt from JRiver has said this:
"The program requires full control over its library folder, and deletes files in that folder that are obsolete or foreign."The old files in the root of "D:\Films" are probably now deleted. Any subfolders in there should still be intact.
I hope you have backup files. Otherwise you can only try some file recovery tool. For instance:
Recuva. It is free and quite good.
-- Don't do any further changes to your D: drive (don't save, change or delete any files)
-- Download and install Recuva. (Take care that nothing goes to D:. Install it to some other drive.)
-- Read its instructions and run it to find the deleted files.
-- Recover the files to some other drive. (If you don't already have a backup drive this is a good time to purchase an external drive for storing backup copies.)
I hope this helps and you can recover most of your files (some of them may already be overwritten).