MySQL does not generating PID file

I upgraded my OS from redhat 2.6.3 to 2.6.5 after that when I restarted my MySQL server but it not started,

error message is "150320 11:53:17 [ERROR] Error message file '/usr/share/mysql/english/errmsg.sys' had only 641 error messages, but it should contain at least 728 error messages." and when I fired the command service mysql start it giving error "Starting MySQL......The server quit without updating PID file (/var/mysql-data/hostname.pid). [FAILED]"

Please help..