After you file

For what tax year--current 2018 or earlier?   Does the letter have a CP or LTR number in the right corner?

Is there any claim made in the letter about what they are doing or alleging?   It could even be at random.  Did you file a return for the year in question?

Did you purchase  Audit Defense for the tax year in question?   If, so we can tell you how to contact the company that handles that.

 If not, you can phone TurboTax free Audit Support Center for help with an IRS letter.   They close in 40 minutes today (at 5 PM Pacific), but reopen on Monday.
To get the phone number go to this link and click the blue Get Help button and follow the prompts.
<a rel="nofollow" target="_blank" href="https://support.turbotax.intuit.com/irs-notice/audit-support/">https://support.turbotax.intuit.com/i...>

You may get suggestions in this forum as well over the weekend.