According to the IRS Understanding Your 5071C Letter webpage, this letter may be sent when the IRS needs more information to verify your identity in order to process your tax return accurately. Receiving this letter DOES NOT necessarily mean your identity has been stolen, but only that the IRS needs to verify your identity prior to completing the processing of your return. You can access the IRS website through the previous link to review information regarding the letter you received and to access the secure IRS Identity Verification for IRS Letter Recipients webpage or call the number provided in your 5071C letter.
IRS Publication 4535 Identity Theft Prevention and Victim Assistance provides information on minimizing the risk of having your identity stolen. It also provides information on the steps to take if you ever believe you have become a victim of identity theft.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.