I assume this is why we had a a power cut at about 2am.
We had (at least) two short power cuts around that time, if the number of local house alarms and electronic equipment resetting is anything to go by. It would be terrible if it was arson, but it could also have been a falling power line starting the fire rather than falling in consequence - I would wait for the police and fire service to confirm.
It is terrible for the stable owners and I hope that other equestrian centres will be able to help them out with temporary storage.