The $1,800 amount sounds like a Recovery Rebate Credit on the 2020 tax return, Form 1040 Line 30.
If you indicated on the 2020 tax return that you did not receive the first stimulus payment of $1,200 and did not receive the second stimulus payment of $600, the total of $1,800 would be the credit and included in the 2020 federal tax refund.
If the IRS has records indicating they sent you both stimulus payments this is why you are now seeing 0 for your tax refund. The IRS would be sending a notice in the mail explaining why the refund has changed.
After receiving the IRS notice and it is because of the Recovery Rebate Credit being removed you can request a trace of the payments if they were never received.
Go to this IRS website for how submit a payment trace - https://www.irs.gov/coronavirus/second-eip-faqs#paymentlost