I received notice from IRS today that in 2018 Paypal reported I earned 40K and I never reported it. This is fraud. How do I go about proving it? I never received 1099k

IRS lists an acct number and my SSN.   I never even used Paypal in 2018 and never earned income.  I'm at a loss how to prove this wasn't me, I'm suspecting fraud and someone using my SSN.