I am so sorry to hear you have lost your cat. Only a PM could ascertain the cause of death really. Had your cat been out all night? Could he have been hit by a car (normally if their claws are "scuffed" or broken this can be an indication). Assuming you have no plants or flowers that may be toxic to cats (i.e. lillies etc) it may perhaps have been a stroke. My only other thought is that is it likely he had bitten an electric cable and received a shock?

Sorry again to hear of your very sad loss.

__________________
Kaz

For kittens, joy is for the asking and smiles are for the giving