If you receive the message that you are temporarily restricted or unable to connect, the first step is to attempt to log onto another realm. For instance, if you are on USWest, attempt to log onto USEast. If you cannot access only one realm then you may have a temporary IP restriction.
The article below covers the most common causes of temporary restrictions.
Keep in mind, anyone behind your network (siblings, family, friends, etc) who also play on Battle.net or if your ISP shares your IP address (as in more than one persons are given the same IP address) and gets restricted from Battle.net, the penalties will be applied to everyone using the shared IP address. This is the unfortunate disadvantage of using a shared IP address.
It is possible that the computer may be infected with a virus that has resulted in the restriction. If this is a concern, run a virus scan.
It is important to realize that these restrictions are not exactly a punishment, but rather that they are necessary for server stability. When you spam, it places a much heavier load on the server and ruins the gameplay experience for everybody. For restrictions other than spam, such as using an invalid CD-Key, you are restricted to prevent abuse like stealing someone else’s CD-Key via brute-force.