SSH Connectivity to a Azure VM stopped working

This VM on Azure had been running (CentOS 6.3) for few months successfully. All of sudden the SSH connectivity stopped working. There are other 2 VMs, which are doing fine.

We re-started the server, shutdown and started, re-created the SSH endpoint. Nothing seem to help.

The error thrown is "Connection closed by <IP Address of VM>".

Any ideas to resolve the problem are welcome.

Thanks.