Nope, it's not just you, it's the whole of Steam that's currently out of action. Valve's digital store front has been inundated with traffic since the Winter Sale launched yesterday (for what it's worth, you can read about some of the biggest deals in the sale by hitting this link) and it seems as though the platform is heaving under the weight of so many customers (either that or it's some sort of DDoS attack). There's no word on when the store will be up and running again, but it's quite likely that there's some people over at Valve HQ working hard to rectify the situation and get things back up and running again.
Update: You can access the Steam Store via a web browser, and via the Steam client. Whether it was a DDoS attack remains unclear, with this appearing on Techworm. We're not sure either way at this stage, and will update you as and when we know more.