Server Not Found
 

Firefox can't find the server at start.ubuntu.com ??? Why can't this system find the Internet?

No idea, especially since you provide no details at all. The website could have been down for a few minutes, your router could have rebooted, the cable could have been loose...who knows? The site is up and available now.

If you still can't get to it, provide details...things like how you're connecting, through what hardware, using what version/distro of Linux, etc.

Ok...

Think in layers.

At the bottom is the physical layer, if you are not connected either by an ethernet cable, or wifi or modem nothing is going to happen.

Check that.

Right, then you need to check the your system knows about it.

Possibly the easiest is to pop up a terminal emulator (command line) and say
ifconfig

Check the results.

You should always have an interface "lo" which is just the local loopback, ignore it.

You need something like a wlan0 or an eth0.

Great.

Can the system talk to that device... well, if it could, it will no the HWAddr MAC address for it.

Is the device set up? If it is it will have an inet addr:

Check all those things first. And then we will have some better ideas.

Or yes, of course, the obvious question: Was it working before if so what changed?