i am using free tier as a new user. i am connecting my EC-2 instance from my laptop through putty.

Hi,

Your problem can be related to incorrect login which varies depending on AMIs. Use following logins on following AMIs:

• ubuntu or root on ubuntu AMIs
• ec2-user on Amazon Linux AMI
• centos on Centos AMI
• debian or root on Debian AMIs
• ec2-user or fedora on Fedora
• ec2-user or root on: RHEL AMI, SUSE AMI, other ones.

If you are using OS:

• Windows - get PEM key from AWS website and generate PPK file using PuttyGen. Then use Putty to use the PPK (select it using left-column: Connection->SSH->Auth: Private key for authorization)

• Linux - run: ssh -i your-ssh-key.pem login@IP-or-DNS

refer- https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/putty.html

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I was doing one terrible mistake. I deleted the original key pair and created the new one that was not accepted by the system. i created new instance and by its original key pair ssh that instance by the steps provided by you its connected. Thank you all for your support