Lollipop - what mode is this?

Hi. My week old smartphone got itself in an odd mode the other night AFTER fully charging to 100%. It basically wouldn't respond - as if the battery had been taken out but it HAD just got to 100% (I was watching it at the time). ** no screen response to any keys **
At this time I plugged it into a PC and device QHSUSB_BULK appeared with 6 drives of which the first (image) could be read. Appeared to have modem s/w in it.
Pic here


Does this tell any experts what mode the phone was in please?

What happened next? I waited 24hours for phone battery to deplete (its a poor battery) then connected to PC again and immediately for "Charging 0%...1%" on phone display and it was back to "normal".

Thanks for any help on this.