First, gather some information. In the letter they sent you, you should see a specific tax year for that "past due IRS tax obligation." That's the first piece of info you'll need to know. Then you'll want to locate a copy of that year's tax return and any previous correspondence you might have received pertaining to it.
Next, visit the TurboTax support page (for notices & audits): https://support.turbotax.intuit.com/irs-notice/audit-support/. That will give you a general understanding of your options. If you purchased Audit Defense, then you'll contact them first, before talking to IRS.
If you didn't purchase Audit Defense, then you'll need to respond to IRS in order to dispute their decision (the above link will show you a sample letter, and an IRS toll-free number). You may also need to make a fact-finding phone call to IRS if you're unable to piece together why they took your refund. They'll tell you the specifics, which you'll want to know before composing a response letter.
