Your mower does not start. Sometimes it fires weakly, but yanks the starter cord from your hand.
Your flywheel key is sheared and it needs to be replaced. The flywheel moved and sheared the key. Now the engine timing is very much incorrect. The spark comes at a time that makes the engine want to run backwards, but it cannot.
As a safety precaution, always remove the spark plug wire before working on a small engine.
Step 1: Why?
The flywheel key shears to protect the crankshaft from twisting, which would cause a very expensive repair. A soft key allows the flywheel to move on the crankshaft when your mower blade hits a tree root, an old stump, a large dried clod of dirt, etc. If the mower blade is loose, that will cause enough shock to shear the flywheel key, too.
Pictured is a repair I made to the blade holder. It had two raised tips to fit the blade slot. Those had become battered and the blade had some looseness, no matter how tight the center bolt was. I drilled out the tips and replaced them with hardened bolts. (Because of space limitations, I had to grind one side of each bolthead away at the top of the blade holder. This blade is actually a thatching blade. Somehow it is causing flywheel keys to shear. I think I will no longer use a thatching blade on this mower.)
Step 2: A new key
Although sold by Sears, my mower engine is made by Tecumseh. There are several flywheel keys used on Tecumseh engines. Even if your mower is sold by Sears, you can take the Sears model number to your local mower shop and they can find the correct key for you. The key shown is part number 611004. It has a wider, thicker portion at one end. That end faces down on my engine. I was assured the key and flywheel would fit only when the key is in the correct position. (The photo is from Google Images.) A new key is about one dollar (US) in cost.