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:

According to some online forums, this is a common problem. The PTFE tube gets deformed some times, due to excessive heat or extruder parts jogging during the print. Mine was so bent that I couldn't even remove it from its base.

Fortunatelly, this is easy to solve: Simply use a drill to remove the clogged plastic,along with the dented parts inside the teflon, remaking the original 2mm hole through the tube: