This is simple enough to diagnose yourself using these steps, assuming you have a gas lawn mower. Don't know much about electrical ones.
1. Disconnect spark plug, and drain gas.
2. Buy a bottle of carbon cleaner ($5)
3. Remove air filter and clean it up by removing all debris. This is the number 1 reason that a mower won't start. Make sure no gas/oil in filter connector area.
4. Remover carburator, and clean it with the cleaner.
5. Before connect spark plug back, clean it first and make sure no oil.
Each mower is different, check your owner manual first. If you haven't done any oil change, I suggest you check if engine oil is too dirty and change oil if necessary.
