Updated 14 July 2022: Normal service was resumed on Twitter shortly after posting, and this article has been amended to reflect such.
Today Twitter went down for users around the world for unknown reasons. The outage began at approximately 4:55am PT / 7:55am ET / 12:55pm BST, and lasted for around an hour, making it the site's longest-lasting downtime in years. Twitter used to be much more unreliable in its earlier days, with outages relatively frequent, but it hasn't seen anything like this since a 2 1/2 hour downtime in 2016.
I've asked Twitter's comms lot what happened, and will update with any new information. Per Twitter Support, some users are still having trouble, but for the majority it's now working fine. If you want to keep an eye on the site's status yourself, try Downdetector.
Obviously, now it's back, the joke of the day will be somehow linking Elon Musk to the downtime. The world's richest man has spent most of this year in a will-he-won't-he attempt to buy the social media giant, which has now ended up with him pulling the plug on the deal, and Twitter suing him. Musk originally offered to buy Twitter for $43.4 billion in April, after which he spent a lot of time complaining about bots and posting poop emojis (I can confirm the poop emoji has now been cited in court documents).