After months of good work, the quality of my 3D prints started decaying until there was no extrusion at all.
This is usually caused by a clogged nozzle: low quality impure ABS filaments leave traces of other material sticked to the nozzle channel. You can solve this easily by leaving the nozzle soaked on acetone overnight.
But after I removed the nozzle to clean it, I realized that I sill couldn't extrude. So, I disassembled the whole extruder and saw that the fault was on the teflon (PTFE) tube, as you can see on the following image: