As the title suggested I have lost my Linux Instance password. I have already accessed it via putty and it's ppk key. But I don't remember the password of system.
Is there any another way to access the system again?
Your screenshot says "server refused our key". That means you don't have the private key for your instance. I'm not sure there is a password.
You can try something like this to mount the volume then reset the password / create a new key.
As far as I know, there is no passwords set by default. Important is that you try to login with the correct user.
Depending on which AMI you selected this can vary.
ec2-user for AWS Linux,
ubuntu for Ubuntu,
centos for CentOS.
If you feel that you cannot get access to system. You can create AMI of your instance, create a volume of that AMI and then create a new instance and mount that volume on new instance and recover the data.