Why won't my lawn mower start at all?

you don't say if it electric or pull start.

What's happening with it? Do you choke it? Do you need to prime it first??For any engine to run you need three things.

Compression, fuel and spark. Is your fuel old. replace it with good gas. Check your plug, make sure it is not fouled. Then the last thing, if it is a pull mower you should be able to feel the resistance when pulling the starter gear. On electric , how is your battery? Have checked your connections and made sure there is good contact?

Start there and judging by the question after checking these things the next would be a local lawn mower fix it place.