You don't really need the PIN you used last year; you can make up a new one.
For purposes of signing your electronic return, you can reuse your Self-Select PIN from last year, or you create a new one to use. It doesn't matter, so you don't need to find the PIN you used last year.
Please see the TurboTax Help article What is the difference between the five-digit Self-Select PIN I created last year and my Self-Select... for more information.
The IRS requires this information to help verify your identity.
