You can find the password from EC2 management console System Log.

1. Go to Amazon EC2 console at https://console.aws.amazon.com/ec2/.
2. Chose your Instance and Click Actions → Instance Settings → Get System log

Get System log

You can also SSH into your instance and find the login credential using the following command.

$ cat /home/centos/credentials 
////////////////////credentials info////////////////////
--- phpMyAdmin
----- Setting  : Success
--- MariaDB
----- User     : root
----- Password : lkb3kM5lfjntvnHa
////////////////////////////////////////////////////////