How do I retrieve my tax id number if I lost or can't remember it?

Is this for a business? Or your social security number?

You should have a paper trial for when you first got it, and if you every paid taxes associated with that tax id # it would be listed on the forms.

Follow wintergirl, and if unsuccessful
If it is your social security number, then go to the nearest social security office with photo ID and birth certificate.
If it is your ITIN, then contact IRS at 1-800-829-1040.

All advice given is solid!

I have nothing to add.