If you used TurboTax to prepare your income tax returns, you may be covered by the TurboTax Audit Support Guarantee. You may also be able to get assistance with the notice you received.
Please note that the IRS does make mistakes. It is a good idea to carefully review your notice and return to make sure that the additional taxes they have assessed are legitimately owed. If they are requesting additional information, you should provide everything that they have asked for by the response date indicated on the letter.
For more information on the audit support guarantee: