auntification requirments changed unexpectedly

I have our intranet site running on IIS6, with 'integrated windows' and
'basic' security enabled. SSL is enabled. Users inside and outside the
domain were able to log on with username and password. For no apparent
reason, users now need to use domain\username and password to authenticate
correctly. Some folder permissions were changed but have been changed back
to the original setup. Can anyone please suggest a solution please?