The letter should say what they changed. A common adjustment is if you claimed the Recovery rebate credit on 1040 line 30 when you already got the Stimulus payment or got the second one after you filed or reported the wrong amounts or entered the wrong amounts in the 2 boxes. If you entered the total you got for both rounds in the same box it would think you didn't get the other one. The IRS knows how much they already sent you. You might have tried to claim it again on line 30.
Did they bill you for penalties or interest?
The TurboTax program only computers and estimated underpayment penalty based on the information you entered in the program as of the due date of the return. However the IRS has the right to bill you for any more underpayment penalty, failure to pay penalty, failure to file timely penalty and interest based on when you actually made the payments.