In FoxPro tables, each .dbf file can have a matching .cdx (or maybe
  a .idx) file for its index(es).  The corruption or invalidity would
  be related to that file.

  If it's a FoxPro table, and the index is "corrupted", it might be
  because:

  - Maybe the index expression is too long or has a typing mistake in it.
  - Maybe there are too many indexes attached to the physical .dbf file,
    so the .cdx or .idx file becomes too large or too complex.
  - Sometimes, an index gets "corrupted" during run-time and nobody knows
    why.  So if I go into FoxPro and say on FoxPro command line:
                  use table_name exclusive
                  reindex
                  close
                  quit

    That usually fixes the "corruption".
 

  If it's a FoxPro table, and the index is "invalid", it might be because:

  - Maybe the field you pick as your "index field" for the Delphi Ttable
    is not a key field or not a primary index field in the physical
    database.  You might need to go to FoxPro command line and create an
    index that starts with the field(s) you want to use for indexing:
                 use table_name exclusive
                 index field_name tag index_name
                 reindex
                 close

      If you want to have more than one field in your new index, change
      the 2nd line to plural like this (you can have + more than 2
      fields):
              index 1st_field_name+2nd_field_name tag index_name