It almost sounds like the blade is loose, the blade acts as a flywheel to the engine, check the blade bolt is tight, tip the mower back on its handles, remove the plug cap, check the bolt tension, normally a 9/16 spanner. If all ok it may have sheared the flywheel key, you can usually tell if either of these things are an issue because the you will get a kick back on the starter handle.
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It almost sounds like the blade is loose, the blade acts as a flywheel to the engine, check the blade bolt is tight, tip the mower back on its handles, remove the plug cap, check the bolt tension, normally a 9/16 spanner. If all ok it may have sheared the flywheel key, you can usually tell if either of these things are an issue because the you will get a kick back on the starter handle.