Follow the instructions in that notice to verify your identity ...

I got a 5071C letter from the IRS

The 5071C letter means that the IRS has received your return, but needs more information from you to verify your identity.

You can do either of the following:

• Go to Identity Verification Service and follow the online instructions.
• Call the IRS toll-free on 800-830-5084 for Identity Verification.

(For either option, you’ll need a copy of your prior year tax return, current return, and supporting documents like W-2s, 1099s, or Schedule C)

Or did you receive an IRS Letter 5747C in the mail?  If so, see this IRS website for information - https://www.irs.gov/individuals/understanding-your-letter-5747c

Or was it an IRS Letter 4883C?  If so, then see this IRS website for information - https://www.irs.gov/individuals/understanding-your-4883c-letter