Steemit itself was in read-only mode for a few hours earlier today. It was giving a message about "maintenance work". It didn't indicate whether this was planned maintenance or an error.
My guess is that, maybe, something went wrong with the blockchain, or the miners, or the witnesses or whatever, and that affected all the sites that run on Steem.
Steemit is now back up, but Steemd isn't, as you've noticed. I don't know any more.
I am pretty sure that some threshold or time limit had been reached and they updated the codebase of the website. I know that at least one change was made to the codebase, and when I saw these messages I guessed that it was an update of the scripts on the webserver.
I raised an issue on the github about the way the 'mute' function works, and within a day a developer started a discussion on the thread and observed that the way it worked, or rather, didn't really work, was actually caused by a bug. So it is likely that this bug was one of the things that required this 'read only' mode to be set. The blockchain itself cannot be unilaterally stopped like this, it was just the website.
Changes to the website code that write to the blockchain, the site cannot be allowing any of this code to run as it is being changed. Code that affects only reads from the blockchain, when these files are modified, it may cause an odd glitch for the unlucky person whose browser makes a request for it in between when the beginning and ending of writing the new file happens, but then anyone after that write would see the new version. This does not impact on the state of the blockchain, so read access does not have to be disabled. However, write access does have to be disabled for a code update.
I hope that clarifies what was going on. I did not pay close attention to how many pending issues were on the github, you can see the repository that stores the working codebase that the website runs on here: https://github.com/steemit/