I was travelling today and didn't have proper access to the server, or I would have been on it sooner.
We may have run out of some resource. In that case it may be hard to know for sure what happened.
Edit: this was using https, I didn't try http.
The reason it caught my attention was due to the recent https "Ask" threads, and my first thought was perhaps full time https had been implemented. I wouldn't bring it up except that it was unusual and correlated with HN content and the outage, even though it is likely to be a coincidence.
I didn't personally notice any http outage or delay.
"But there is no question that it is a good idea to get to the bottom of why this happened. I have found in the past it always pays to do a post-mortem on any system problem as many times there is something lurking that will cause that same problem in the future." 
I can't tell you how many times over the years some little thing has been an indicator of something else going on or caused a big problem later. It's normally better to diagnose this while you are not under duress, have room to breath, and aren't under any pressure.
 Non system related last week my wife couldn't get into the garage because the GFI on the same circuit as the garage door opener had tripped. The same GFI is in the upstairs bathroom and had blown previously many times but it didn't seem to be a problem. We didn't realize it also controlled the garage door. So she couldn't get into the house (she didn't have her key). So by ignoring the problem (just keep resetting the GFI and hey - figure it out later) we had a larger impact and issue than it seemed on the surface.
It's been sitting on the front page for the past few hours. I suspect several sources led to the TC article and then through to the post on HN.
edit: removed the 'A'
I wouldn't exactly call that immense but more importantly it an increase like that certainly doesn't seem to be a reason for a site like this to go down (I didn't see it was down I'm going by the headline).
For example: HN relies heavily on caching page data. If 70,000 robots suddenly started loading random items, it could easily cause both the jump in unique IPs and the downtime.
The obvious question is isn't there anyone else who minds this tremendously popular site and is on duty if you are traveling?
"Despite being immensely popular, it is a labor of love and generates little (or no?) revenue, so unfortunately, no there isn't anyone else minding the site."
Frankly, every time HN is down, I wonder whether this is finally the day PG has given up on the project, because it's slipped into dire form in recent years.
What do you define as "harm"?
People are creatures of habit. If something isn't available many times they will fork and find something else to do with their time which they might find as or more enjoyable or educational and then start to gravitate toward using that website. You only have so much time. If you find an alternative site that you like that might very well end up being your "addiction" and then that site could build critical mass.
I used to start the morning with my laptop at Starbucks. I no longer do that. For the longest time I did that every morning. Then something changed and I found something else to do in the AM rather than that and achieved the same benefit. As an aside this is well known by Starbucks. I was told by one manager that at all costs they try to keep a SB being renovated open lest they loose people who have made it a habit to stop by and pickup their coffee.
Now all this may or may not be important to PG. And sure a little downtime isn't going to have a major impact on usage.
But there is no question that it is a good idea to get to the bottom of why this happened. I have found in the past it always pays to do a post-mortem on any system problem as many times there is something lurking that will cause that same problem in the future.
"Hacker News goes down. Thousands of developers forced into unwanted productivity."