If you can only not connect to one realm, then it is an IP restriction. There are two kinds:
1.) Temporary - lasts up to 12 hours, usually caused by too many repeated actions (entering/leaving games too quickly, for example)
2.) Extended - Lasts exactly 2 weeks. You have this if you can't connect to that realm for over 12 hours. With very few exceptions, this is caused by trying to connect with an illegal CD-key onto BNet.
All you can do is wait it out.
2. Re: cannot connect to b.net? 09/19/2011 01:45:21 PM PDT
The IP address has warranted a lengthy restriction and will not be able to access Battle.net for another 13 days.