Wired internet connection problems - Diagnosing the problem
First things first - turn your modem/router off for a few minutes and then power it back on again. Don't assume that this step is too obvious to prove effective; this simple move solves most internet connection problems and should be your first attempt at getting back online.
Not sure what your 'router' is? It's the black box with flashing lights - see below - which manages your internet connection (wired or wireless). It's also worth contacting your network provider to make sure that the service isn't down (in other words, that it hasn't stopped working) in your locality.
If the issue persists, try the following: