The first think you need to check is around the blade to make sure there is nothing stopping it from turning. Wear some leather work gloves when dealing the blade to help prevent any cuts that may occur.
If the blade looks to be clear, take the spark plug out and then see if you are able to get the engine to turn when you pull on the starter. If you are, check the oil to make sure it does not smell like gas. If it does, you will need to drain all the oil (with gas mixed in) out of the engine and dispose of it properly. Then you will need to clean the carburetor. Next, you will need the tip the mower to where it's only on the front wheels to drain any gas/oil from the head. After that, replace the spark plug with a new one, put fresh oil in, fuel the mower up and cut grass. The mower will smoke for about 20 minutes after this repair due to the residual oil in the head.
If the oil does not smell like gas, is it low? If not, with the spark plug out, tip the mower and see if you are able to turn the blade by hand. You will need to have someone hold down the lever that you hold to start/kill the mower. If you are still unable to get the engine to spin there is something bad wrong with the engine and you will need to have a qualified technician take a look at the machine to determine the problem.
If you found gas in the oil and you need some direction for how to clean the carburetor, let me know back on this thread and I will assist you in getting that taken care of. If that does turn out to be the case, I will also need the model number the lawn mower so I can determine which engine you have and which carburetor it has on it.
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