I know it's been answered, but it wasn't answered very well.
Basically, the server that PC is located on is constantly taking requests for member's posting, and even just viewing. The more requests, the more the server load. Servers have a "cap" or maximum load they can take. Once the server load reaches it's cap, it denies all requests for until the server load is at a steady and safe level.
In other words, PC's server hit it's maximum load, and you were trying to access it, so it denied your request to view the index or whatever thread, profile, blog, anything.
That's also why it helps not to constantly hit the F5 key (refresh button) in Internet Explorer/Firefox/Chrome/Safari/Your browser.
If you request, the server load will deny it and spend more time denying that request than letting the load go down.
I hope that helped.
And to the person who neg-repped me for this:
It's very funny how there wasn't a valid answer, isn't it?
And also, DOES IT REALLY FREAKING MATTER???
The original poster NEVER EVEN said that it was fixed, therefore it wasn't solved.
IN. Your. Face.