Filament stuck in hotend
I was printing too much - so when switching from Hatchbox Glow to Prusa Vanilla White... something went wrong and the printer jammed hard.
The issue turned out to be filament stuck in the nozzle. 
I
I removed the nozzle and heated it in a jar of boiling water. That softened the PLA in the back sufficiently that I could stick a needle into it, let it cool, then pull out the 58mm back section of the jam.
Used boiling water to soften the filament. Got out a 58mm section with a needle.
There still appeared to be filament stuck in at the thread section of the nozzle - even though I could get a 0.4mm wire all the way through the nozzle.
So I reinstalled, heated to 155 C and loaded some PRUSA White ABS. After purging about 20 cm of that, I switched to a cleaning filament and purged about the same amount of that through the printer.
Now I am back printing again.
PUNISHMENT LINES:
... I WILL MAKE SURE MY NEXTRUDER NOZZLE IS FULLY INSERTED INTO THE NEXTRUDER !
... I WILL MAKE SURE MY NEXTRUDER NOZZLE IS FULLY INSERTED INTO THE NEXTRUDER !
... I WILL MAKE SURE MY NEXTRUDER NOZZLE IS FULLY INSERTED INTO THE NEXTRUDER !
... I WILL MAKE SURE MY NEXTRUDER NOZZLE IS FULLY INSERTED INTO THE NEXTRUDER !
... I WILL MAKE SURE MY NEXTRUDER NOZZLE IS FULLY INSERTED INTO THE NEXTRUDER !
... I WILL MAKE SURE MY NEXTRUDER NOZZLE IS FULLY INSERTED INTO THE NEXTRUDER !
... I WILL MAKE SURE MY NEXTRUDER NOZZLE IS FULLY INSERTED INTO THE NEXTRUDER !
... I WILL MAKE SURE MY NEXTRUDER NOZZLE IS FULLY INSERTED INTO THE NEXTRUDER !
(and that the Thumbscrews are TIGHT !!!)