Are you using a router or wireless connection?
Recently, have you changed your PC setting?
I cannot see your PC environment directly, but here are troubleshoots from my anticipation
A. Try temporarily disabling router and connect your modem directly to your computer.
B. Try temporarily disabling any firewall or virus security programs that you are running:
a. Right-click on [My Network Places]
b. Select [Properties]
c. Click [Local Network Connection or Modem]
d. Click [Advanced]
e. Uncheck [Internet Connection Firewall]
C. You need to get your port opened
* If you are on WINDOW 7, please click following linkhttp://us.battle.net/support/en/article/opening-ports-for-the-blizzard-downloader-on-windows-7-firewall
D. If you are running Windows 7, please right-click the game icon and choose Run as administrator.
E. Try launching the game from selective startup mode:
Hit the Windows Key + R, and type: msconfig
Hit Enter or OK.
In the General tab:
1. Choose Selective Startup
2. Disable (uncheck) Load Startup Items
3. Click OK and Reboot
F. Check LAN settings:
If you didn't install or set up any proxy settings, go to Internet Explorer -> File, then make sure Work Offline isn't checked.
Also check Internet Explorer -> Tools -> Internet Options -> Connection -> LAN Settings and make sure your proxy information is correct.
If you don't use a proxy, make sure that everything is unchecked and hit OK
Hope this would help you and thank you for your efforts in advance :-)