I’ve come across this issue twice in one week so here it is ?
After you have done the initial SQL installation of SQL 2008R2 and added the SP2. Everything might seem fine – it can actually be fine as well!
However, you might also encounter that the server assessment from the ConfigMgr 2012 splash screen says everything is fine.
Then you start your Configuration Manager setup and it fails to connect to the SQL db.
If you try telnet the ConfigMgr server on port 1433, It’ll probably fail to connect.
Try to run netstat -a to see if the port is showing in the list.
If not; start up the SQL Server Configuration Manager.
Go down to the SQL Server Network Configuration and verify if TCP/IP has status Disabled/Enabled.
If disabled, enable it and restart the SQL Server Service
Run netstat -a once more to verify that the port is showing
You can also try to telnet to it once again to verify that it connects.
Now you’ll be able to continue your ConfigMgr setup ?