If your network connection is not working, think small. Check the network cable that connects your computer. There should be an activity light on the RJ45 interface where the network cable plugs in to the computer. Nothing will break network connections quicker than someone stepping on your network cable and pulling it out of the RJ45 plug. Make sure that the network cable is plugged in securely on both ends, and that the activitly LED on the computer is lit.
Linux, Unix and Windows systems come with diagnostic tools to help with network troubleshooting.
On Windows, hit the Windows key + R to open the Run box. Type CMD in the run box and enter, which should open a Windows command prompt, which looks like a >.
On Linux, log in to the computer. That should take you to a $, # or > prompt. If your Linux system boots into a Gnome graphical interface, you can open a terminal from the Application, System menu.
From the prompt, type "ping www.cisco.com". You should receive 4 replies from www.cisco.com. If Cisco answers you, you have an network connection and an internet connection. On Unix and Linux systems, the default for ping is to keep pinging until you interrupt it. To make ping try ten times, enter this "ping -c10 www.cisco.com". The "c" in the command is for count. If you don't put in a count on Linux, you can stop ping by hitting the CNTL + C keys. On Windows, you can type "ping -n 10 www.cisco.com" to ping ten times, "c" on Unix, "n" on Windows.
If you do not receive replies from www.cisco.com, you may still have a network connection, but possibly not internet service. To check your internet connection, on Windows at the > prompt, type "ipconfig". This should return a number of lines including one called "Default Gateway". To find your default gateway on a Linux system, type "route". The "default" line has the ip address of your default gateway. The default gateway is your "next hop" toward the wider world. This is normally your router, or a possibly a switch on larger networks. If you just have one pc connected to a cable or dsl modem, it might be a device at your cable or telephone company. Test the ping command against the ip address of the default gateway. If it works, you may need to call your internet service provider for support. If it does not work, check the default gateway device and make sure it is powered on and has activity on its led indicators.
Have you installed a new router, switch or cable modem lately? Routers and some cable modems function as dhcp servers. A dhcp server allocates and tracks dynamic ip addresses on your network. Two dhcp servers on the same network will effectively shut down your network. If you got a new router, turn off your old router. If you got a new cable modem, try turning off your router and see if you still have an internet connection.
Rebooting is usually worthwhile, particularly with Windows systems. Cable modems and home routers also often benefit from a reboot. This can fix the problem of duplicate ip addresses, as well. There cannot be two devices with the same ip address on the same network.
Whatever you do, do not start changing settings. The only way changing settings is going to fix a broken network connection is if you changed the settings to start with. The proper way to troubleshoot network problems is by starting at the simplest physical connection along the line and working your way up. If your cat pulled out your cat 5 cable, it is easy to fix by plugging the cat 5 cable back in. Unless you changed a bunch of your router settings before you looked at the cable.