A stopped nozzle results in underextrusion and in many cases some unpleasant noises from the extruder. The clog can occur very suddenly or it can build up over a long period of time, but they often end up in the same way - a completely useless print. These clogges are often caused by dust getting into the hotend assembely which can't get out beacause of the small diameter of the nozzle. To prevent this, a dustfilter can be printed, filled with some sponge and fitted to the filament. This will dust the filament off before it enters the extruder. But should it happen to you anyway here are some simple tricks to solve that problem.
Perform a cold pull
The first place you can see signs of a stuffed nozzle is in the infill. Due to the slightly higher speeds, there is a higher pressure in the hotend which has difficulties getting out if something is stuck in the nozzle.
It's a good idea to clean the nozzle with a cold pull which is performed by heating the nozzle up to the print temperature of the material mounted on the printer. When hot, push some filament through before setting the printer to cool. When it reaches 50 degrees, the heat can be turned on again and then pull on the filament until it releases from the hotend and takes impurities out of there. Do the same 2-3 times more to be safe. For best results, use filament such as nylon or PETG for this as they are a bit more slippery than standard PLA.
Replace the nozzle
Heat the print head up to about 180 degrees (for PLA - use the melt temperature of whichever filament you use) and pull out the filament slightly before the nozzle can be unscrewed with a small wrench (some printers come with a wrench the right size for the nozzle on your printer). Don't touch the nozzle while it's hot. When the nozzle is unscrewed, place it on something that can't burn, and find your new nozzle (some printers come with extra nozzles in the package). The new nozzle can be screwed in at the same temperature as the other one was taken out (again without touching the nozzle). Tighten the nozzle lightly and do not force it too much, as you can break the nozzle inside the heating block.
Burn out the impurity
Proceed with caution; this involves using open fire near your hands - only do this if you're comfortable with it, and you have control over the situation. Don't let children perform this (sidenote; this can also smell quite a bit)
Heat the printhead up to about 180 degrees (or whatever temperature is required for your filament to loosen) and pull out the filament slightly before the nozzle can be unscrewed with a small wrench. Watch your fingers! Don't touch the nozzle! Hold the nozzle with a pair of pliers so that the threaded side faces downwards and allows the plastic to run out and take the impurity with it. Hold up a lighter on the nozzle, and let it heat up until the plastic becomes so liquid that it can even run out. You can stop when you can see light coming through the small hole in the nozzle. The nozzle can be screwed in again at the same temperature (again, without touching the nozzle. This can be frustrating but it is very hot after heating) Tighten the nozzle lightly and do not force too much as you can break the nozzle inside the heating block.