>Can the indexes be corrupted ?

I suppose - but I have never seen it.


>How can I fix this corruption ?

Assuming there is corruption, you need to drop and recreate the index.


>
the sales table has 11 mill rows. if I add one index it takes about 10 hours to run.
>

I am assuming you already had a PK. Also, did you selected a clustered index? When you run the query against the optimzer, what does it suggest?