Do you have the form 8606 from when you made the non-deductible contribution?
If not, then you pay tax on the entire withdrawal. You need to have saved the form 8606 to prove that you made a non-deductible contribution. You get a new form any year that you make a non-deductible contribution, or do a Roth conversion, or withdraw from a Roth. The information carries over and you need the most recent form 8606 even if it was a long time ago. Without it, you can't prove the non-deductible contribution and everything will be taxed (the IRS doesn't keep track for you.) The IRS may have your tax returns including any form 8606 going back up to 10 year, but not farther.
https://www.irs.gov/individuals/get-transcript
If you have the form 8606, the you will use it to report the withdrawal. You must withdraw and pay tax proportionately. For example,if 2% of the balance is your prior non-deductible contribution, then 2% of the withdrawal is not taxable, and your non-deductible basis is slightly reduced. You will get a new form 8606 as part of your tax return that you will use the following year to keep track of your after-tax basis, and so on into the future until all the funds are withdrawn and taxed or converted to a Roth and taxed.
