Check for any grass stuck between blades. Also if the engine does not engage when the motor spins,the gear is probably broken. Clean the carburator and check for air leaks or improper adjustment. Examine the lawn mower's spark plug.
If your Craftsman lawn mower isn't working, troubleshooting ideas would include, checking the spark plugs, the oil levels, and whether there is gasoline in the mower. Check to make certain the wheels are at the proper cutting levels. Also read the manual, to see if you're operating the mower properly.