The filament is not correctly inserted into the hot end and cannot be pushed properly by the extruder. Remove the filament, make a sharp cut on the end of the filament and insert it again into the hot end.
The filament diameter is not compatible with the diameter of your hot end and cannot fit inside the body of the hot end.
The cooling of the plastic from the previous print might have inflated the plastic inside the hot end preventing the extruder to properly feed the hot end nozzle. You will need to extract the filament from the hot-end, cut the end of it and push it back to the hot end.
The hobbed bolt used to pushe the filement inside the extruder might be full of plastic and so is sweeping instead of pushing the plastic filament. This can happen if the extruder screws are too tight and so the hobbed bolt is just smashing the plastic filament. At this moment the plastic dust will fill up the teeth of the hobbed bolt. You will need to clean it up by either blowing air into it, or unmount it and clean it with some brush or needle and clean the teeth one by one.