Did you or anyone misclick or try 6x wrong passwords to log into the admin? After 6x wrong passwords have been added, the account ends up locked. It will be visible for you as an administrator in the module people under Active users with the “locked” tag.
Next steps to navigate a user what to do:
Click on ‘Forgot password’
Confirm it by clicking on the “Reset password” button
Go to your email box and click on the link to create a new password
