There was no sign of injury, and it didn't look emaciated or otherwise ill, so my guess is that either it died of old age or poison. We've seen it pottering around for the past few weekends, so poison seems unlikely.
It's a shame: I like hedgehogs, and we're running out of them because people keep flattening them on roads.
I shovelled up the corpse and dumped it on the wood pile at the bottom of the garden. Something down there will want to eat it.