HI Amber,
To isolate the issue you should try and login to client tools like webi rich client or in CCM (manage servers) using your AD credentials.
Case 1: Login to client tool works:
Your CMC config and service account is ok.
Check the tomcat stdout.log file to find any errors related to kerberos.
Also, check the krb5.ini file.
Case 2: Login to client tool doesnt work:
If not, then you might want to run a check on the SPN entered in CMC --> Authentication --> Windows AD.
You can also check the password validity of service account using the kinit test (steps provided by Ajay).
Also, Start a trace on CMC and try and make this work first before going for web client login.
Regards,
Jatin