I lost the original video files. I still have the .fcpbundle file. This file is 16 GB in size, so I think it contains the original files. There are many large files in the folder 'High Quality Media'. Are these the original files and can I open them?
In FCP if you select one of the missing clips (which are probably red), then select File>Relink>Original Media -- but do NOT click anything on that dialog -- at the bottom it will show the pathname FCP is expecting to see the file.
If that is within an .fcpbundle, it is internal and should be available. If the pathname points to someplace on disk *outside* the library where that file formerly was, then it was imported with "leave files in place". The only thing left in the library are symlinks to the former file locations. In that case the 16GB size of the library is probably cache files which are not useful in this situation.
If you ever created proxies, those should still exist within the library. They will be lower resolution but should match frame-for-frame to the original media.
If none of that works and you are running Time Machine, the files may be backed up there.
If the volume is HFS+ (MacOS Extended Journaled) and if the data is vital, I'd suggest discontinuing any write operations on the disk and consider data recovery. There are 3rd party tools like Disk Warrior, also commercial data recovery services. Those only potentially work on HFS+, not APFS.
None of these are the original media. In the library bundle there are two High Quality Media folders. One is in Transcoded Media, and the other is in Render Files. The files in Transcoded media can be opened; the files in Render Files cannot.