A dirty carburetor, bad gasoline, failed spark plug or broken recoil starter can prevent the tiller engine from starting.
Stale gas won't start the engine. If you left gas in the fuel tank during the winter without adding fuel stabilizer, drain the tank and fill it with fresh gas. Add fuel stabilizer to gas to keep it fresh during the off-season.
Bad gasoline can clog the carburetor, so clean and rebuild the carburetor if the tiller doesn't start after replacing the gas in the fuel tank. Replace the carburetor if it's too clogged to clean.
A dirty spark plug won't ignite the fuel to start the engine. Complete a full tune-up that includes replacing the spark plug, air filter and oil.