IT may be related to either the QC server <-> LDAP server connectivity or something do do wtih the DNS cache.

See this HP article (you will need an HP ''passport'' login to see it) about flushing DNS cache:

https://softwaresupport.hp.com/group/softwaresupport/search-result/-/facetsearch/document/KM00368094

More likely, this is related to your LDAP settings in QC.
Since you cannot login (but could previously), it leads me to believe that something changed about your LDAP user or ldap server used by QC.

You will need to get into Site Admin tool to fix this, but as long as it is set to LDAP, you will NEVER be able to login.

So, you must have the DBA help you to reset the parameter in the qcsiteadmin_db database PARAMS table back to ''QualityCenter''.

Once this is done and you cycle your QC server/services, you can login with your Site Admin user that was used previously before you enabled LDAP.
All users have a stored password in QC - either their password that was used previous to turning on LDAP or possibly ''blank''.
Try logging in as the Site Admin User with a blank password or if you remember an old password (QC-Authentication, use that).

Then when you are in Site Admin, SCRUTINIZE the LDAP settings (Site Users tab/Authentication Settings and LDAP Import) and confirm that the Directory Provider URL is correct (see QC Admin Guide Chapter on Managing Users).