The same thing just happened to me with my L4400. I was grading my road when my boys came by in the golf cart to tell me something. I shut the tractor off to talk to them and then, nothing.
The Service guys from Kubota were there this afternoon and even he was baffeled. We tried to jump it at the starter and even that didn't work.
With only 35hrs on the meter, they loaded it on the rollback this afternoon to take it into the shop. When I find out what it was, I'll post it for future owners.