Have you seen the following troubleshooting information in the EC2 User Guide? https://docs.aws.amazon.com/en_us/AWSEC2/latest/UserGuide/TroubleshootingInstancesConnecting.html
Check the section that talks specifically about "Connection timed out": https://docs.aws.amazon.com/en_us/AWSEC2/latest/UserGuide/TroubleshootingInstancesConnecting.html#TroubleshootingInstancesConnectionTimeout
It's not clear to me whether you've successfully connected to this instance or another instance half a year ago? If you connected to this instance, have you stopped and then started your instance since you last connected to it? When you stop your instance, the public IPv4 address that Amazon EC2 automatically assigned to the instance on launch or start is lost. When you start your instance, a new public IPv4 address is assigned. Check that this IP address appears in your connection command.
I hope the above helps!
Kind regards, Marilyn
It turned out that I mistakenly deleted the internet gateway attached to the VPC of that instance. So re-creating an internet gateway and attaching it to the VPC again solved the connection issue.
By the way, I meant that I could connect other instances a half year ago, not the one I deal with now. But they were inside the same VPC.
The provided link reminded me to refer the route of VPC, that was the cause of this issue.
Thank you for letting me know :-)
Best wishes, Ryosuke
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