Printer stopped extruding on a filament change midway through a print. Figured, nozzle clog and was only minimally annoyed because that happens some time. Except the nozzle needle didn’t clear it. So I took the hot end off and did the allen-key trick to pull out the plug. Reassembled, retried, still no joy. OK, I was printing Nylon which should be fairly immune to heat creep, but that happens too. Take the hot end off again, confirm there’s no filament in it. Take the extruder off, take it apart. Nothing. No filament at all. Bad stepper motor? No, I remember on the last test that I confirmed the yellow gear was turning.
Then, I remembered, when I backed the screw out to release the cutter lever, the lever was in the pushed-in position. It had gotten stuck so the blade was in the way of the filament. Duh.