On Sun, 4 Jul 2010 22:15:32 -0700 (PDT), harry wrote:

IT sounds like the engine is unbalanced
Yes. Something went unbalanced but I checked (visually only) the blade
itself which seems to be all nicked up but no worse than it has been for a
loooong time.

therefor some rotating part has broken off or the crankshaft is bent.
It could be a bent crankshaft. I guess. But, I've hit far worse than what
made this happen this time as it happened all of a sudden on me.

How would I know if the crankshaft is bent?

Or possibly the engine has come loose on it's mountings.
I didn't think of that. Will check tomorrow but I didn't notice anything
loose as I flipped the lawn mower over to look at the blade.

The obvious one would be the cutter blade. Check that it's running
without a wobble ie not bent.
It doesn't look bent. It doesn't look like it's wobbling. Would that
prevent it from restarting? It ran for a few minutes "after the accident"
but roughly - and now it just won't start.

Or, as it won't start something associated with the igntion system.
Could be. I don't see how that would be associated with hitting something
but as I said, it wasn't all that much that I hit (I've run over far worse
with that mower) so maybe something just broke in the ignition. I can pull
the plug and check.