According to the IRS website Understanding Your 5071C Letter, 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 link above to review information regarding the letter you received and to access their secure Identity Verification Service website 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.