处于睡眠状态的Linux Apache

I have a Linux Ubuntu server having Apache2 that runs PHP. All of a sudden my website gets error:

404 Not Found: The requested URL / was not found on this server

I tried:

sudo service apache2 status

Result:

Apache2 is running (pid 1546)

So I tried:

cat /proc/1546/status

Result:

State: S(sleeping)

Apache error.log file:

[Fri Jul 24 05:22:46 2015] [error] [client 155.124.123.161] File does not exist: /etc/apache2/htdocs

My question is, is this maybe the reason why my Apache server is not working properly that I get Not Found error? Is Sleep state of Apache a problem and what should I do about it? My website codes is in path /var/www/mywebapplication. It is working for a very long time but suddenly it gets error.

Thank you very much.