You asked, What's meant by index?
An index is a set of pointers to your records. All databases use them, so
they can locate records instantly. Before computers, you could go to any
library and find 3 indexes for the books: one by author, one by title, and
one by topic. They were literally index cards. You could look up an author,
and find the key value (e.g. the Dewey number) that indicated where the book
was physically stored, and so find the book without having to look through
every shelf in the library.
In table design view in Access, each field has a property called Indexed (in
the lower pane.) If you choose Yes, Access maintains an index on this field.
Now if you sort by this field, or search on this field, Access can find the
value instantly without having to scan the entire table to find the match.
For the sake of performance, these indexes are maintained in RAM, and only
written to the disk periodically. If the power goes off during a write, the
index can contain invalid or incomplete data. A symptom of this can be that
the index only goes as far as B, so all the later records appear to be
missing when Access is using this index. However, if you sort differently,
so Access uses another index, all the records suddenly turn up again.
All databases uses indexes, and there is always a way to tell them to
rebuild their indexes. In Access, that's part of what the "repair" does
under:
Tools | Database Utilities | Compact/Repair
Sometimes it takes a bit more than that to fix the index, particularly where
other tables are connecting to this one based on that index. The article
explains how to handle those steps.
Of course, it is also possible that something else went wrong, e.g. the data
was actually deleted by someone, or a pointer to the actual data went bad,
or your disk sectors are cross linked. But the problem with the index is
more likely, so fingers crossed.
