You didn't write in which state you leave m92, if you do not use it.
Is is switched on (you can read the text) or in sleep mode (there is a screensaver on the display) or switched off (blank screen - I guess, I do not use it.)
It could be related to WiFi, which consumes quite a lot. Didn't you leave WiFi on after leaving the browser?
M92 becomes unresponsive to pen (and buttons) if the battery is exhausted. So, it could be opposite also in your case. The battery was exhausted and so stylus and M92 did nothing (but the text was visible, as it does not need the power supply). It has happened to me once or twice. First I was scared, something broke, but than realized it only needs to recharge. So, it is normal behaviour for e-ink.
It might be a good idea to reset M92 to screensaver Battery Empty or something like that when power is close to low. It is (IMO) better to go to sleep sooner than to scare the reader that something has been broken.