If for seemingly no good reason you can't open Server App (Lion) and/or Server Admin then follow the below instructions. Essentially just check if servermgrd is loaded with launchctl

From here: http://support.apple.com/kb/TS3960

To check if servermgrd is loaded, see the output of this command in Terminal:

sudo launchctl list | grep -q com.apple.servermgrd && echo loaded || echo not loaded

If the output is "not loaded", load the com.apple.servermgrd job into launchd with this command:

sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.servermgrd.plist