This is a subject covered in many other forums but didn't see it here, so I'll summarize.
Earlier this week I was mowing the backyard when I went over a hidden piece of wood. It stopped the engine dead and bent the blade about 3" out of plane, enough for it to drag on the ground. Focusing on the obvious, I set out to replace the blade and found one at Sears (Craftsman brand mower).
After replacing the blade, I pulled to restart it, and pulled, and pulled. After about the 15th time it barely caught and started to run but much slower. Just cutting the lawn caused the engine rpms to drop and since I had some high grass, it made for very slow going. I then noticed that the muffler was glowing red and said, "that ain't good"...as if everything else that happend before was normal, right?
So, I figured I'd killed it and in my excitement and frustration went to search for another mower to replace it which I did and brought it home.
The next day, I got my wits about me and searched the Internet for similar situations. Turns out that this is quite common after hitting an imovable object and the damage can range from minimal to severe.
If you are lucky like me, then all that has happened is the flywheel that also triggers the ignition coil has moved (advanced) in relation to the crankshaft. They are kept in sync by a small aluminum key that gets sheared when the engine stops abruptly but the flywheel's momentum causes it to continue turning. It had become offset by about 30 degrees. All it takes to fix it is a new $3 key.
More severe cases will bend the crankshaft and render it useless. This can be checked by tipping the mower up and having someone pull the cord with the spark plug disconnected. If the center bolt wobbles, then the crank is bent.
Other sites go into detail on how to replace the keey but it is as simple as disassembling the top of the mower, and removing the large nut (23mm in my case) that holds the flywheel to the crankshaft. I used a large 3-jaw puller to pop it loose but a steering wheel puller would work too.
Clean the broken pieces out, scuff the surfaces with emery paper and replace the flywheel, lining the cuts up with the crankshaft keyway. Drop in the new little key (made of aluminum) and retorque the retaining nut. Reassemble, refuel, and give it a yank. Hopefully, it'll be good as new.
Hope this helps!