Iona,
I presume this is a push mower? If the mower struck a solid object will running, there is a key that can shear off to prevent the crankshaft from damage. It is on the bottom side of the mower where the blade mounts to the shaft. Hold the blade in place, remove the large nut and washer, then remove the blade. Check the shaft to see if the key is damaged. Some don't have a key. Some blades have a mounting hole designed to shear as to avoid damage to the crankshaft. If this is the case, a new blade is needed.
Good Luck!
Gunny 63
June 2010
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