Filament can get jammed inside of the 3D printer's hot end. This can be due to pieces of dirt falling into the printing system. Improper storage of filament can mean that excess moisture causes the plastic to warp and perform poorly. Airwolf has a guide that walks through how to remove a jam from the printer head.
Once the jam has been removed, it is HIGHLY recommended that the user then performs a proper Hot End Nozzle Cleaning, which is outlined in another article.