Totally mystified.

Last Monday I went to Bourne End on the Velo. I rode it home fine and nothing seemed unusual or untoward. Left it parked outside. On Friday I went to go out on it to discover it seems as if the engine has totally seized. Can't kick it over. Tried bumping it....no joy. Clutch works. Gears seem to select Ok and find neutral. How does a bike seize when it is sleeping??

I shall disconnect the primary drive first in the hope that there is a problem there. Then I suppose I will have to start working down from the valve gear. Not a happy bunny :-(