It does happen on occasion when your pull cord doesn’t move and it seems locked up. The pull starter could be bad or the engine carburetor could be leaking fuel down into the cylinder and hydro locking it up.
You can try to remove the spark plug and gently pull on the starter rope to see if it clears the fuel from the cylinder. Make sure to place a rag over the cylinder head to catch any gas that may spill out of it.
Also check to be sure that something is not interfering with the lawn mower blade that would keep it from spinning such as a tree branch or rock.
Check the oil. If it is very low, it is possible that the engine has locked up due to low engine oil and it could be ruined. If you don’t see any oil on the dipstick, that could mean the end of that lawn mower.
Now would be a good time to take your Self-Propelled lawn mower to a lawn mower mechanic to have it professionally checked out. You can find one on our locations page.