CentOS 7 stopped accepting connections today.
I found ypbind was not bound so it could not access the NIS database to get user authentication information.
However, ypbind would not rebind on restart. Usually the cause of this is portmapper (rpcbind) not running. I checked, it was but I restarted it anyway, still ypbind would not bind.
At this point I attempted to reboot the machine, systemd got stuck during shutdown and would not complete the shutdown process so I was forced to hard boot it.
There were two newer kernels than the one it was running on. It is now running on the newest and seems to be operating normally again.