If you are not connected, your network may not have a DHCP service (DHCP is responsible for assigning network settings). In this case, you will have to manually enter your network settings.
Manually entering your network settings
If you don't have DHCP on your network then you can configure a static connection. You will need to enter the network settings yourself, so check with your network administrator or look at your router's settings to find out which details to use.
1. Right click the Network Manager icon in the system notification area and click Edit Connections.
2. Click the Wired tab, select the connection and click Edit.
3. Click the IPv4 Settings tab and choose Manual from the Method drop down list.
4. Click Add and enter your IP address and other details. Enter the address of your DNS server too.
5. Click Apply. The network should now connect if you entered the settings correctly.