When you get the promised notice you will know what they changed. Was there an amount on the form 1040 line 30? Was that amount your entire tax refund ? If so most likely the IRS declined the credit ...
The letter from the IRS probably is stating that you indicated on our 2020 federal tax return that you did NOT receive a stimulus payment and therefor the amount that you did not receive was entered on the Form 1040 Line 30 as a Recovery Rebate Credit.
However, the IRS removed the credit from your tax return because the IRS shows that the stimulus payment was sent to you. By removing the credit this reduces your federal tax refund or increases the taxes owed.
Go to the IRS website for your tax account to see what the IRS has sent to you for stimulus payments - https://www.irs.gov/payments/view-your-tax-account
If you never received the stimulus payment shown on your account then have the IRS put a trace on the payment. Go to this IRS website for how to start a trace - https://www.irs.gov/newsroom/questions-and-answers-about-the-first-economic-impact-payment-topic-f-p...
