I just discovered a major mistake in my year 2000 US federal tax. As a consequence I overpaid my year 2004 tax (quite a lot of money). Is there anyway I can get my money back or is it too late.

Walpi:

It's too late by over a year. Statutory law requires any amendment that results in a refund must be filed no later than three years after the normal filing deadline for the year in question.

Since the 2000 return filing deadline had to be filed by 15 April 2001, you had to file the amendment not later than 15 April 2004.

Sorry.