There are a variety of reasons why this can happen, and rather than spell them all out with the hopes that one will stick, I'll refer you to http://dev.mysql.com/doc/mysql/en/can-not-connect-to-server.html in the MySQL Online Reference Manual.
In a nutshell, make sure the server is running, make sure the post is not firewalled, and if connecting with unix sockets, make sure the socket file has permissions for writing.
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