Could you please check if you have APE-Tags in your files and also check if you read them?
If you read them, then please switch that option off and see if the data comes back again
The tag panel is empty if there is not at least 1 selected file. Loaded files are not enough, you have to select it/them.
You can set in the options Ctrl-O to select all files automatically when opening MP3tag:
Please note that MP3tag also works on several (selected) files in one go. You can edit e.g. all files of an album at once and set the ALBUMARTIST and YEAR and ALBUM once and save that data to all files without repeated entries.