Did you earn the income from a Connecticut source, such as working in Connecticut, conducting a business in Connecticut, or selling property in Connecticut? If so, you have to pay Connecticut tax on it even though you live in another state.
What year were you audited for? Did you live in Connecticut for part of that year? Is the issue a question of allocating income between Connecticut and another state?
What was the nature of the income that Connecticut wants you to pay tax on? Did the auditor say why you have to pay Connecticut tax on the income?