Regarding Keypair

While launching the instance key pair is required or not ?

Strictly speaking no, a keypair isn't required as a prerequisite for an instance being launched. Although if a keypair is not provided then in most cases you won't be able to login to the EC2 instance, unless the AMI provides another method - check the documentation for the AMI.

And if the keypair is lost what are the possible ways to recover/login to access the instance

The process for replacing a lost keypair with a new one is here https://repost.aws/knowledge-center/user-data-replace-key-pair-ec2

And the associated video that walks you through the process is here https://www.youtube.com/watch?v=5V2hKFj9_SI