I am due a refund on an old return I have not filed. What can I do to get it?
You must file that return within 3 years of the date it was originally due, including an extension if you filed one. This is known as the refund statute of limitations. But, there is a refinement of that rule that prohibits refunds of payments made more than 2 years before the claim for refund is made once the 3-year period has expired.
Note that the application of these rules can be quite complex and sometimes baffling - even for tax professionals.
Also read How to correct mistakes on tax return