The retraction value is too high and the PLA can expand inside the hot-end
The filament diameter tolerance is too low, which can cause jamming inside the bowden plastic tube or even on the hot end itself
Extruder screws are tighten and deteriorates the plastic filament causing a jamming inside the hot end
Some dust or other similar particles are blocking the hot-end nozzle causing the plastic to be stuck inside it.
The fan blowing on the full metal hot end heat sink is stuck or stopped causing the PLA filament to inflate inside it.
First, it’s strongly advised to first remove the filament from the hot end while it’s still hot.
If you cannot remove it by retracting the filament using the big wheel of the extruder, you will have to remove the PTFE bowden tube so that you have direct access to the filament at the base of the hot end. Do to this, you need to unplug the pneumatic connector at the base of the hot end and attempt to remove manually the filament by hand or using the flat pliers if you struggle too much. You might require to extrude the filament a little bit to that it makes it possible to remove the PTFE tubing from the pneumatic connector. In the case where there are some impurities inside the nozzle you will have to cleanup your nozzle.