I used my lawn mower three weeks ago and it was working perfectly. Today when I saw that is that time again to use it. It won't start. The propeller seems to be stucked. I opened and clean it but it won't start when I pull the string... any suggestions? Thanks in advance for your help.

Um, define the propeller please, are you referring to the blade, or the recoil starter on the top of the engine?

Could be the brake cable, or could have too much fuel in the cylinder. Move the small handle at the top of the handle bar and try to pull the rope, if it is tight, remove the 1/4" headed screws and see if the brake is moving. Your brake cable may be broken. Another thing if the brake is operating, remove the spark plug and see if the mower engine will turn over. It (the cylinder) may be full of fuel.