Individual files from my project are missing and cannot be found in the backup folder either

It is likely that this file has been stored in your external files folder and the link has been broken. When a file beyond a certain size (default is 5mb) is imported, it is stored and read from an external folder to limit project file size.

In your main MAXQDA window, under the "Home" tab is an option for "external Files." If you click "list," you'll find all your external files listed in the table along with their filepath and status. Here you can check if the connection is working properly, or if the file has been moved from the external files folder MAXQDA is trying to source from.

You can read more about this functionality here.