I wonder if perhaps it could be related to a concerted major hacking attack against Amazon. In this day and age it would not surprise me to see a foreign, or even American, group try to make a statement or simply attempt to disrupt a major American business on a key day for them. I doubt this is the case though. I suspect it was simply Amazon not being adequately prepared--as usual.
I was still having major issues with the Amazon USA site even late into the evening. If I knew what I wanted to look up I could type that specific item and it usually found it. But trying to browse deals was a joke. For example, I typed "SD card" into the search box under the Prime Day category and it returned either the doggie pictures and "sorry" message or it returned a page showing about 400 pages of hits, but only the first page would display, all the rest returned doggie pictures and "sorry" message! Cute dogs though.
I haven't tried much today, but so far no doggie pics. I did manage to find and buy a Sandisk 32GB Extreme Pro SD card, but of course it wasn't on sale. Most of the sale items I looked at were not very good deals, and many had inflated list prices. But as I recall, this is typical for Amazon's Prime Day event--mostly hype with very few good deals. The best deals are around Black Friday and Monday and during Lightning sales.