Head gasket leaks
if the headgasket has gone, coolant may have gone in to the cylinders preventing combustion
temporary sealant will not cure a blown out headgasket, will make it worse
Will the engine turn over and just not start? If it won't even turn over anymore you may have a cylinder full of coolant. Pull all the sparkplugs and see if it will then turn over. If you do have a cylinder full of coolant then you know that the magic head gasket fixer in a can was a waste of money.
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