Bad Loan Written Off in 2020 Business Tax Return 1120S. However in 2021, the loan started to perform - what do I do?

You wrote it off as a bad debt? You can amend and that would include the K-1s and any shareholder would have to amend their personal returns.

In addition to ammending the 2020 form 1120-S, you may also have to issue corrected K-1's, and the recipient's of those K-1's may need to also amend their 2020 personal 1040 tax returns. As for amending the personal tax returns, it only needs to be done if the corrected K-1's will make a difference to their tax liability, or change any NOL's or carry over's.

you may even have written off the wrong amount in 2020.  usually, the reason the shareholder loan can't be repaid is because of losses.  losses first reduce your basis in the stock then to your basis in the loan.