Apache stops responding but is running

A few things that could cause that are:

Possible Causes:
1) MaxClients set to a value too low
2) If you have over about 800 sites, the ErrorLog files open too many file descriptors and apache won't be able to log the errors and may stop responding.

Solutions:
1) edit /etc/httpd/conf/httpd.conf and increase the MaxClients setting to something like 200 or 300.
2)

cd /usr/local/directadmin/data/templates
cp virtual_host*.conf custom
cd custom
# remove all the ErrorLog lines (or comment them out) from the 4 virtual_host*.conf files that are in the custom directory.

echo "action=rewrite&value=httpd" >> /usr/local/directadmin/data/task.queue
Apache should be restarted automatically after a few minutes later (rewrite might take a while with over 800 sites).

3) Other possible information:
Edit /usr/include/bits/typesizes.h and set
#define __FD_SETSIZE 32768
and then recompile with customapache.

On FreeBSD, it's /usr/include/sys/select.h
Change:
#define FD_SETSIZE 1024U
to
#define FD_SETSIZE 32768U

then recompile apache/php

4) edit /etc/sysctl.conf and add:
fs.file-max = 32768

and run:

/sbin/sysctl -w fs.file-max=32768
then recompile apache/php
Related error messages:
[error] System: Too many open files in system (errno: 23)

host: isc_socket_create: not enough free resources socket.c:2117: REQUIRE(maxfd <= (int)1024) failed.
host: isc_socket_create: not enough free resources