When it comes up, does it give you "Unable to Connect to Battle.net"? Are you able to connect to other realms?

If so, that is merely an IP ban. They generally don't last more than an hour. What usually provokes that, is too many connections to battle.net in a short amount of time (usually while xferring/pindle runs). I believe it is merely a measure of protection versus a DDOS attack. Or alternately, it could be an anti-botting measure.

If not, please describe exactly what you see, or better yet, supply a screenshot.