What should I do if a CP14 says I did not pay but my account shows zero?

Treat it as a reconciliation problem first, not as proof that you still owe the tax. If the IRS account already shows 0, the most likely issue is that the notice cycle was triggered before the payment fully posted.

A practical response sequence is:

1. save proof of payment
2. save a screenshot or transcript showing the current account balance
3. compare the payment date, posting date, and notice date
4. respond only if the account later stops matching the payment history

Do not pay the amount a second time just because the notice is alarming. The better question is whether the account is now correct. If the account still shows a balance later, then respond with the payment proof and the timeline.