Often when a program or process crashes, there is a stale pid file left behind. When you try restarting it, it thinks another instance is running.
Log out of Gnome and log into one of the virtual terminals. Then delete the contents of /tmp. Then reboot.
If there was a failure that caused networkmanager to fail, you may need to discover what it is if it persists. For example, I had the onboard NIC fail on my old laptop. Look in your logs and see if there was a Network Manager message or a kernel message at the time of the failure.
A general fix is to reconfigure your network devices. This might remove any harmful changes you may have made since the failure.
If you don't have wireless, using network manager is kind of pointless, unless you have different network profiles, such as you might with a laptop you take home with you, and one location doesn't use DHCP.