Are you referring to the IRS IP PIN which is 6 digits long? If so you have to get your IP PIN from the IRS : https://www.irs.gov/identity-theft-fraud-scams/retrieve-your-ip-pin
Or the 5 digit PIN to efile the return? If this one you can pick any 5 digit number to use as your filing signature ... you do not need the one you used last year.
