Well, ordinarily, I’d say, no, it’s not logical for things to just stop working. However, there are a number of reasons that might happen.
A good place to start checking on the status and health of the PHP engine is with phpinfo():
https://php.net/manual/en/function.phpinfo.php
You should see that mbstring is enabled.
The Simple HTML DOM third-party component used in the extension merely calls mb_detect_encoding() which itself is a function provided by the mbstring PHP extension. So, what you are seeing is likely only a symptom of the problem, and not a problem within the plugin itself (if you were to run something else on the server, such as phpMyAdmin, you would probably receive the same error, as phpMyAdmin also requires mbstring to be enabled).
What probably happened is that something is either keeping the mbstring extension from being loaded (or found), has removed it from the list of extensions to load in php.ini, or the server is running out of resources somehow, making it impossible for mbstring to function.
Start your research on the basic error message reported:
Call to undefined function mb_detect_encoding()
There are lots of search engine hits for this issue. Check the php error log, assuming you have it enabled, and also the web server error log for some clues. In any event, this isn’t something we can address in the plugin code. We need the Simple HTML DOM parser component to dissect the elements on the page to hand off to the PDF engine. Simple HTML DOM has a dependency upon mbstring. If mbstring’s functions aren’t available, that’s what needs to get fixed.