Some mp3 files that I open in MP3Tag do not show tags for length (duration) nor bitrate.
This information is available in Windows File Explorer, right click the mp3 and choose properties.
This information is available when listening to the song in WinAmp, right click the mp3 and choose View File Info.
This is header information. If there is none, then this may be the indication that the header has been corrupted.
See if one of the utilities linked in this "HowTo" thread helps you: