First, detach the SQL server from the farm by running the "SharePoint Product Configuration" and in the SQL Server- > New Query, run these command to repair the SharePoint config database.
1.EXEC sp_resetstatus SharePoint_Config;
2.ALTER DATABASE yourDBname SET EMERGENCY
3.DBCC checkdb('SharePoint_Config')
4.ALTER DATABASE SharePoint_Config SET SINGLE_USER WITH ROLLBACK IMMEDIATE
5.DBCC CheckDB ('SharePoint_Config', REPAIR_ALLOW_DATA_LOSS)
6.ALTER DATABASE SharePoint_Config SET MULTI_USER
And now the config database repaired and returns the warning and exception.Again ran the "SharePoint Product Config Wizard" to attach config database (alternatively we can also say joining the farm).
You need to give the pass phrase while attaching the config database to SharePoint server,
Now configuration started
After the wizard open Central Admin
When you get the http 500 Internal server error, look all services, app pools and IIS settings, all of them should be up and running.
But interesting thing was all SharePoint sites are opening except "Central Administrator" site.
Run the STSADM updatefarmcredentials with due diligence and IIS reset.
Now you should be able to see the "Central Administrator"
reference : https://social.technet.microsoft.com/wiki/contents/articles/32422.recovering-the-suspected-sharepoint-2013-config-database.aspx
