I cut the grass about 4 weeks ago with this mower Honda 3011 and had no problem whatsoever. Yesterday when I tried to start, it won't start but I heard just clicking sound. I don't understand what could happen. What could be wrong? Any help is appreciated. I must admit that I am mechanically ignorant.

Try charging your battery first. Maybe you didn't shut it off or the battery is at the end of its life.