I have Plex in more than 1 year, and not happened before.
After I created New Libary and with point folder where my videos are for that Libary, nothing showing even if i run Scan libary files. Weird
Plex usually can’t find files or is intentionally ignoring ones that are not named correctly. it intentionally ignores files names like tv shows in a movie library and vice versa. So if the file has like S01E01 in them then a movie library will ignore. if it does not have that then a TV library will ignore.
It is possible that other numbering schemes might get it confused. You scanner and PMS log should have info about the files. and if it is ignoring them or not.
Of course it could be something like file permissions but that usually happens on linux, not common on Windows.
9/10 this issue is either down to naming convention issues or permissions issues. Use Filebot (unfortunately you have to pay for it these days) to bulk rename all of your movies/tv shows correctly, alternatively you have to do what BigWheel has stated and name them manually in that, specific formal. Plex is a stickler for correct naming conventions.
After that, make sure that all files and folders have the correct permissions set. Assuming you have a Windows-based server (based on your tags), I think that the minimum you need is “Read & Execute”, but you could try starting it at “Full Control”, and gradually reducing the permissions until it stops working. At which point go back to the last one tried. Alternatively, do a search on forums as someone somewhere must have figured this out already! Ha.