Lawnmower starter cord - seized

Yes. Take the spark plug out & try turning it

Check that the engine flywheel brake is fully engaged, this is the bar on the handle that you hold down to run engine, when released it stops the engine. It could be stuck half ways or faulty. That is assuming your mower has such a bar !

Make sure there isn't anything caught up in the blade to stop the engine turning.

String picked up from the lawn or a heap of long grass. Had to clear off something

similar to that from a neighbours mower a few weeks ago. I'd be surprised if it

is seized. If Honda made people, nobody would die :-)

if the blade is hard to turn with the plug out then either something has warped or there is something stuck in there, as above, twine or wire can wrap itself bloody tightly around these things.

Mowers have pins that are designed to shear should the blade hit something immovable/solid. This means you dont break the engine or the blade, and typically just need a new mounting bracket/flange/connector.

There is also a woodruff key within the engine that would shear to again save the engine/shaft. if this was broken the blade would turn freely (the engine would also start, but the blade wouldnt move!)

You might have a damaged cylinder or connector rod, but this would be strange enough to happen without someone noticing.

I believe that mower has a 4 stroke engine, is there oil in it?!

I used to work contract cutting grass with a gang of lunatics that could and would damage any mower and didn't give a toss what they mowed over. I've seen plenty of damage to mower engines but I've never seen a bent crankshaft.

However what I've seen a lot of after blades hitting things like curb stones is broken flywheel keys. To save further explanation here's a youtube video that might help…

Never seen one this bad often you can hardly notice the damage until you remove the key when you see its slightly deformed.

https://www.youtube.com/shorts/xgLJKoYzjCQ

This one shows the sort of damage that will often create the issue the OP is getting

Same as the issue already mentioned by GreeBo.

A lot of the time you can hardly notice that the key is damaged so you need to remove the flywheel to be sure.