What happened to my Blackberry?

For three days global service Blackberry outage hit the US, and cause millions of people who use blackberry to not have access to emails or use their phones. Research In Motion (RIM) the developer promised all emails would be delivered as the backlog clears. David Yach, RIM CTO said that " that our priority is to get services up and running." The company did not explain what cause it's network to fail.