The simple answer is that you have lost the internet connection, or your computer thinks it has. Why is much harder! It could be any number of faults in your system: modem/drivers/poor phone connections (waggle the plugs at the back of the computer and in the wall phone socket), or the ISP, or noise on the phone line (need to contact your phone provider).
To be honest, I think internet connections overal are pretty poor (stand by for some debate on that one!). I have good days and bad, connection speeds between 28k and 44k, dropped connections etc! It's bad at the moment because I suspect the systems are being bombarded with the new virus (I'm losing count now, but I am approaching 100 infected emails and others on this site are reporting similar).
Try to determine the fault by a process of elimination. Could you get another computer onto your phone line for example?
Pick up your phone (when not on the internet) and see if you can hear any crackles or other noises etc etc
Not too much help I'm afraid, but there may be a few clues!