It could be many things that lead to file corruption. Player issues, file transfer problems, drive failures, and other problems can all lead to this. Mp3tag now reports damaged files and will not allow further actions until the file can be repaired. This is to prevent further possible data loss.
See this thread for suggestions to help repair if possible.
If these tools don't help you may need to re-rip from CD or go back to the original source for an undamaged copy.