I don't know what happened suddenly. MySQL is down and restarting it shows:
mysql has failed, please contact the sysadmin (result was "Warning, no valid mysql.sock file found.mysql has failed").
Fedora Core 1
cPanel + WHM
ln -s /var/lib/mysql/mysql.sock /tmp/mysql.sock
/scripts/restartsrv_mysql --check (showed error that file not found)