Jammed extruder

As I noticed my printer starts printing air and couldn't pull back the filament after cancelling the job, I have taken the extruder out from the print and it seems this half melted PLA basic filament is the cause.

At first I thought it was an one time accident but this is the 2nd time it happened within a week, what could be the cause? Clogged nozzle? Broken heating element?