If it is this switch you get very close to zero reaction when turning the key the last step to engage startermotor

You could try in N instead of P if it happens again. Or as in my case, where this safety circuit failed more severely: add a new earth to the starter relay...

 

I tried that and also tried starting in neutral. Nothing seemed to help then suddenly it started and now seems fine. Cats have nine lives I guess I'm down to eight!!!