If you want to do middle-of-the-night shopping at the Apple Store, it looks like it's down right now. It's offline here, in the US, and some countries like Australia and the UK. It comes with a notice that says, "We'll be right back." And it promises that "updates are coming to the Apple Store." But it still seems to be live in countries like China, New Zealand, and Japan, so it's unclear what this maintenance is for.
Apple typically takes down the Apple Store when it plans to release a new product, but that's typically a worldwide "shutdown." It's most likely just maintenance. But if you're wondering why you can't shop right now, well, the site isn't down just for you. And maybe by the time you see this, the site might already be up.