In the process of 3D printing, it is possible that the hot tip on the nozzle fails to heat up, and this results in the filament not be able to be removed from the nozzle. In other words, we have a clogged nozzle with the extended filament (see picture).

Clogged Nozzle when the heater fails
The nozzle tip failed to heat up.

In this case, we should remove the nozzle after it has been cooled down. The nozzle can be removed from the nozzle over a lighted stove (see video).