Kohler ZT 740

Changed the oil with Kohler oil and filter, ran twice for at least 4 plus hours with no problem,600 plus hours on motor and no problems! Today started and ran for 10 minutes started knocking then seized, still full of oil! Any body have problems like this or heard of any?? My background is in small engine and equipment repair, had shop for 20 plus years and continued working on them to this day and this is the first engine I lost in 60 years so I'm at a loss, any help will be appreciated!