It's happening everywhere: http://downdetector.com/
Ideally, all of those graphs should be (mostly) flat lines.
The short version of what happened is best explained in this Tweet: https://twitter.com/TheProtestBoard/status/499270694702972928
Basically, it's not a WordPress problem, it's a world-wide routing problem, and everyone is affected.
In a more layman explanation, everything has its limits, even the routers which control most of the internet traffic in the world. Today, that limit was reached, and system admins everywhere are scrambling to increase that maximum.
Here's a much more detailed explanation: http://www.zdnet.com/internet-hiccups-today-youre-not-alone-heres-why-7000032566/
I imagine it will be corrected over the next few days, but contact your ISP for a better estimate.