While it is little earlier than expected, the Apple Store is currently in a maintenance mode ahead of today’s Apple Event. Apple tends to do this right before the event kicks off so it is a bit of a surprise for it to be down some six hours prior to the event.
You can still navigate to the site but if you try to buy something, you will get the now familiar “Be right back” page that Apple usually puts up on the site when it is updating it with new products.
If history is any indicator, the site should be back up after the product announcements today. Generally speaking, the iPhone line up gets announced around 40 minutes into the keynote. The Apple Watches will likely be around an hour into the keynote. We should see the site back up for you to buy your Apple goodies.
That said, don’t expect to be able to buy the new iPhone Xs, iPhone Xs Max, or iPhone Xr today. They likely will go on sale at the end of the week. When the new Apple Watch Series 4 will be available is not know but it too will likely go on sale later this week.