Signs of blocked nozzle when filament are not extruding uniformly, extrude very thin filament than usual or not coming out from nozzle.
What you need: (It will cost you less than $15)
1. Acetone ($7 for small bottle)
2. Torch (Available on eBay at around $5)
3. Very thin wire
Important note: Please read your user manual before removing it from the block. Some 3D printers require heat to remove and install.
Step 1: Soak the removed nozzle into acetone for about 15 minutes to clean out exterior dirt. Use a tissue paper/cloth wipe clean the nozzle.
Step 2: Place a nozzle on a stone and burn it using the torch for about 1 min. Make sure it is extremely hot until you see slight changes in the colour.
Step 3: Use a very thin wire to clear the hole in the nozzle. If the wire cannot go through repeat step 2 again until the can go through. You do not want to scratch/damage the internal wall of the nozzle. Do not force through the hole with the wire. I use soft copper wire stripped from an unused phone cable.
If everything goes well, the nozzle should extrude nice smooth thick filament.