Happened to me last weekend too...
When I issue an ps -gaux|grep http I just could see two httpd-a about admin version of httpd. So I issue the same command on another SME 5.5 Server and saw a dozen of http!
After a lot of research I discovered that my httpd.conf files has twice this line (near the bottom) : Listen 0.0.0.0:80
When I commented out one of those lines, I coudl restart httpd process with service httpd start.
If you haven´t even admin pages available you can use : service httpd-admin start to get it back!
Good luck!
Jáder
PS.: Also could to be your ISP blocking 80...