Fixing mysql problem after upgrading 12.04

I have Open Conference System (OCS) installed in my laptop. I use this local OCS for trial purpose before implementing any customized settings to the real server. 

After upgrading my Ubuntu to 12.04, my OCS does not work. Every time I type localhost/ocs in the browser it says:

Database Connection Fails! 

I can make mysql back as usual after re-configuring mysql.

Step 1 - Reconfigure mysql-server

Open Ubuntu terminal: Ctrl+Alt+T

~$ sudo dpkg-reconfigure mysql-server-5.5

Step 2 - Check mysql running normally

~$ sudo service mysql status

the command shows running status: 

mysql start/running, process 6459