You can reset the application to use ALM/QC authentication by having a DBA modify the Site Admin schema directly. Refer to this KM on the HP Support site.
https://softwaresupport.hp.com/group/softwaresupport/search-result/-/facetsearch/document/KM773019

From your case description I understand that LDAP Authentication wasn't fully implemented, and now users cannot login.

In order to revert the changes please follow these steps. Also if you are using Oracle Database make sure that the Commit command was execute, otherwise the changes won't take effect.

On the other hand instead of changing the values manually, please try using the SQL queries below. (make sure that the name doesn't have blank spaces or uppercase in all the letters)

Revert the ''LDAP'' authentication back to ''QualityCenter'':

1. Stop QC Service.
2. Go into the Site Admin schema directly in the database server and modify the PARAMS table with this SQL Statement: UPDATE td.PARAMS SET PARAM_VALUE = ''QualityCenter'' WHERE PARAM_NAME=''AUTHENTICATION'';
3. Look for records where PARAM_NAME starts with 'LDAP', and delete all of them except one 'LDAP_SEARCH_USER_CRITERIA'.
4. Restart QC service again