You can find the password from EC2 management console System Log.
- Go to Amazon EC2 console at https://console.aws.amazon.com/ec2/.
- Chose your Instance and click Actions → Monitor and troubleshoot → 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 : ****************
////////////////////////////////////////////////////////