Thanks for the replies, everyone.

I finally fixed it by taking the hot end off the extruder block, holding it with a pair of pliers, heating it up to ABS printing temp, and using an allen key to push a chipped part of filament through the nozzle. When there was nothing coming out, I cooled it and shook it. A small, green and black beady thing fell out, then it worked fine. Chinese filament, eh?