First open Terminal and log in as root.
# su

After that go the correct folder.
# cd /etc/NetworkManager/system-connections/
Now take a look at the content of this folder. If you had a VPN connection e.g. there must be a file with the name of that connection.

# ls -la
Now you can remove that file or you can move it to another folder (so you can set it back if this solution does not work for your problem).

To move the file to your personal folder use the following command:
# mv <filename> /home/<username>/ #username是你的用户名


Now the only thing left is starting the Network Manager:
# NetworkManager