server not connect ssh

Without more information about the error you are receiving, I have a few questions.

1. What error are you getting when attempting to connect via SSH?
2. Did the IP address change? Check to ensure you are trying to SSH to the correct IP.

"UPSTREAM_NOT_FOUND [519]" can occur for several reasons.
Please check the following documents for common causes of occurrence and troubleshooting methods.
https://repost.aws/knowledge-center/lightsail-resolve-ssh-console-errors

Please check the solutions listed here as they may also be helpful.
https://stackoverflow.com/questions/64243153/aws-lightsail-ssh-says-upstream-not-found-and-also-not-able-to-connect-by-putty