ROTFLMFAO!!!
What if you were hosting a nice little cocktail party and all of a sudden a couple million people from all over the world showed up?
The site is currently overloaded with traffic, so your session is being denied in an attempt to keep it from crashing entirely. The site can handle about 1,300 sessions at a time right now. Unfortunately you are not one of those 1,300, so please try again later.* Better luck next time!
In the meantime, maybe you could play some online poker... I hear it's a great way to win a lot of money.
* Please Note: "Try again later" does not mean "sit there mashing the 'reload' button 50 times a second" because if everyone does that, it' s just going to take that much longer for the load on the server to ease up.