Looks like socket issue. Apache uses them so parent and child processes can communicate. Using the ipcs -a command, you can see a list of all the semaphores that were causing the problem.

[root@localhost ~]# ipcs -a

------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x6c030690 98304 zabbix 600 527280 6

------ Semaphore Arrays --------
key semid owner perms nsems
0x00000000 0 root 600 1
0x00000000 32769 root 600 1
0x00000000 4980738 apache 600 1
0x00000000 5013507 apache 600 1
0x7a030690 29261828 zabbix 600 13
0x530310b0 18219013 root 600 103
0x480310b0 18251782 root 600 9

The following command can be used in CentOS/RHEL to clear all of the apache-related semaphores.

for i in `ipcs -s | awk '/apache/ {print $2}'`; do (ipcrm -s $i); done
ipcrm -m 0x63637069

Now you can try restarting Apache and all done.

[root@localhost httpd]# service httpd start
Starting httpd: [ OK ]