Event 4768 for one user

Hi all, We have two domain controllers running server 2019 with Domain Functional Level 2016. We have one user who has been with us for almost a year, and they were suddenly getting a message in the last week that username or password were incorrect even though we confimed they were not. They were able to initally switch to another user and re-enter their username and log in, but now it won’t allow that.