If the network manager icon is not appearing, but it has been, than it can be fixed using the following commands typed into a terminal:[just copy & paste the lines in green to a terminal :)]
(Change current directory to the home directory):
(Backup in case anything goes wrong!):
mv .gnome .gnome-bak && mv .gnome2 .gnome2-bak && mv .gconf .gconf-bak && mv .gconfd .gconfd-bak && mv .metacity .metacity-bak
Remove the current config files:
rm -rv .gnome .gnome2 .gconf .gconfd .metacity
Now reboot your system(or just simply log out and log back in),when you are done with it you should have the default setup back!