This error took place after the maintenance was performed on my mother server. At first my website showed it lost connection from my database. I was just trying to restart my mysql. Then I had the following error.

[mysql-generic-5.5.40 restart]
=============================================================
mysql-generic-5.5.40
[OK] mysql-generic-5.5.40 is already installed.
ERROR! MySQL server PID file could not be found!
Starting MySQL. ERROR! The server quit without updating PID file (/home/usrdata/mysql-generic-5.5.40/mysql.pid).

cd /etc
rm my.cnf
amh mysql restart

This error occurred because the mother server generated a my.cnf file after the maintenance. The file is incompatible with the my.cnf file which specified from AMH.