why does the piston lock on my lawn mower??

Answer 1

You need to be more specific.
If you remove the spark-plug and attempt to turn the engine, does it still feel tight or does it move freely? If you tilted the engine too far you may have allowed the cylinder to fill with engine oil. If however the piston is still locked, you may have a scorched cylinder wall . Did you hit a large object and bend the crankshaft. What led up to this situation? You may also have a broken valve, that could be lodged in the cylinder and blocking the pistons path- with the spark plug removed, does the engine move freely in either direction ( as determined by turning the blade)?More info would be helpful...

Answer 2

You may have ran it low on oil, you could have hit something and bent the crankshaft.