Impossible to connect to EC2 instance after mountpoint (connection refused)

I tried three times today to reduce my EBS volume following some tutorials that I found on internet. It's a secondary volume, not the root one. Everything seems to work fine until I stop the machine and try to start it again.

Before stopping it, I mount a new smaller volume, rsync the files from one volume to another, unmount both of them and mount the new one in the good directory. When I d a lsblk, everything is perfect then I stop the instance to detach the old bigger volume and I can't start the instance anymore.

I saw many different solutions on internet for that that mainly consists on launching another instance, detaching and attaching again volumes. However, I would like to know why this happens??

The message: ssh: connect to host myhost port 22: Connection refused

Thanks!