The IRS only issues IP pins in select cases, More info.
If you know you need an IP PIN to file your taxes and you lost your CP01A letter (or never received it), you can retrieve your IP PIN online at the IRS's Get an IP PIN site.
You can also call the IRS at 1-800-908-4490 weekdays from 7:00am to 7:00pm local time (Hawaii and Alaska, use Pacific time) to have your IP PIN reissued by mail. However, this can take up to 21 days.
Note: If you don't have your pin you can paper-file your 2016 taxes without your IP PIN. The IRS will review your return to make sure it belongs to you.