The keypair which you are using must be either wrong or lost and there is no way you can recover the Private key pair once lost. The key is like a password and Amazon don't save the private key for security reasons.
To fix it
Go to the aws management console
1. Stop the instance and create an AMI image of the same. 2. Launch a new instance using the AMI Image created and a new key-pair attached to it. 3. Then assign the elastic IP that was previously assigned to the old instance. 4. If everything works fine remove the old instance.
And hence forth save the XXXX.pem file somewhere online.