If you are referring to the second stimulus payment that would have been reported on your 2020 tax return as a Recovery Rebate Credit if you never received that stimulus payment. Look at your 2020 tax return on Line 30 for the credit.
Go to this IRS website for your tax account to see if the 2nd stimulus payment is entered - https://www.irs.gov/payments/your-online-account
If the account shows as sent by the IRS then you will need to put a trace on the payment.
Go to this IRS website for how to request a trace on the 1st or 2nd stimulus payment - https://www.irs.gov/newsroom/questions-and-answers-about-the-first-economic-impact-payment-topic-f-p...
If your account shows that the stimulus payment was never sent then you will need to file a 2020 amended tax return for the Recovery Rebate Credit.
See this TurboTax support FAQ for amending a 2020 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-2020-return/L3XdA1Pw8_U...
