Perhaps the servers are currently under maintenance. Have a look
here.
If the server you play on is operational, follow these steps.
Click on Start, and then Run.
Type regedit and click Ok.
Click on the + icon beside the HKey_Current_User branch.
Click on the + icon beside the Software branch.
Select the Battle.net branch and press the delete key.
Click on the + icon beside the HKey_Local_Machine branch.
Click on the + icon beside the Software branch.
Select the Battle.net branch and press the delete key.
Delete the "bncache.dat" file in your Diablo II directory.