well, i am assuming you have high speed internet (please state type of connection when having connection probs)... There are many causes for this, i will state the easy cures if it is simple prob and if it dont fix, reinstall game.
First i would reset modem and router if applicable (remember off for 10 seconds then on), then go to your network connections and "repair" your connection (right click ---> select repair on the connection you are using). Simple as that you should be able to connect.
But also ive no idea your connection or OS so if it dont work just reinstall game.
Still dont work --- reformat hard drive, you have virus that hates you