starting sshd: /etc/ssh/sshd_config: permission denied

starting sshd: /etc/ssh/sshd_config: permission denied

I am running a Centos machine in Amazon cloud. Suddenly, I can't ssh into
it. Fortunately, there is R Studio running that includes an ability to run
BASH shell. So, I see in /var/log/boot.log that sshd failed to start.
When I run it from the command line sudo service sshd start I get an error
that Starting sshd: /etc/ssh/sshd_config: Permission denied. I tried to
set sshd_config permissions to either 644 or 600 - but I get the same
error. Also I tried sudo su - and then start service.
And it is not the limitation of the shell itself: I can start httpd
without any problems.
I don't even know what else to try...