Hello Geet,
Are you using LDAP Authentication or QC Authentication? If you are using LDAP, is the account password been recently changed or has the account been locked within the Active Directory at any point. I see you mention Active Directory but this only comes into play if you have LDAP Authentication enabled on the ALM system as it is possible to have ALM using its internal QC Authentication even though the rest of the company products are using LDAP/AD?

Have you looked into the user directory within ALM to see if the account is still active and not been deactivated within ALM (still a possibilty whether using LDAP or QC authentications.

Look at the error being returned when the user attempts to log in and that should help you pinpoint where it might be occurring.

Also one last tip and that would be to see if you are using the same client system language coding as the ALM system was installed to use. I have seen an instance a few times where the ALM system was installed using standard English encoding, yet the user was in the Netherlands and used a local keyboard setting. The standard characters were all the same but due the difference in keyboard layout and special character encoding when the user changed their password and used a special character in the password the ascii coding didn't match and the user was always denied access. Upon figuring out that it was one special character that was the issue, we changed the password and instructed the user to not use that special character in future passwords and log in occurred as expected.

Hope this helps,
Dan