Unfortunately, the apache server has been stopped in server or could not start up again check in your apache error log with tail command,

# /etc/init.d/httpd start
Starting httpd:                                            [FAILED]

Check the error log to figure what is the issue going behind, it may output something like this,
 
# tail -f /var/log/httpd/error_log

Configuration Failed
[Thu Mar 20 11:26:18 2014] [notice] suEXEC mechanism enabled (wrapper: /usr/sbin/suexec)
[Thu Mar 20 11:26:18 2014] [error] (28)No space left on device: Cannot create SSLMutex

So, this log indicates us we are running out of disk space. When allocated resource for semaphores gets full we may get this kind of errors. So we need to
check the semaphores details using below commands

(note : check with apache user may name is apache or httpd or nobody)
 
# ipcs -s | grep apache

0x00000000 0          apache    600        1
0x00000000 32769      apache    600        1
0x00000000 65538      apache    600        1
0x00000000 98307      apache    600        1
0x00000000 131076     apache    600        1
0x00000000 163845     apache    600        1
0x00000000 196614     apache    600        1
0x00000000 229383     apache    600        1
...
....

We can fix this issue by increasing space for semaphores or delete all semaphores to free up disk space. To delete all semaphores, run below command
 
# ipcs -s | grep apache | awk '{print $2}' | xargs ipcrm sem
resource(s) deleted

Now, start apache server.
 
#/etc/init.d/httpd start

Finally! save and reload the sysctl.conf execute the command,
 
# sysctl -p

net.ipv4.ip_forward = 0
net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.default.accept_source_route = 0
kernel.sysrq = 0
kernel.core_uses_pid = 1
net.ipv4.tcp_syncookies = 1
kernel.msgmnb = 65536
kernel.msgmax = 65536
kernel.shmmax = 4294967295
kernel.shmall = 268435456
fs.file-max = 200000
kernel.msgmni = 1024
kernel.sem = 250 256000 32 1024

More information about semaphores: If you want to check sem details has been updated execute the below command,
 
# ipcs -ls
------ Semaphore Limits --------
max number of arrays = 1024
max semaphores per array = 250
max semaphores system wide = 256000
max ops per semop call = 32
semaphore max value = 32767

# cat /proc/sys/kernel/sem
250     256000  32      1024