abrupt sudden reboot shutdown pve 5.1

hi all earlier today at 9:42am experienced a sudden abrupt shutdown & reboot, unexplained .. any clues fr anyone? the hardware is beefy, Dell T710 server grade chassis, Raid6, 2 Hexacores and 24GB of ram, it's always been reliable and ran great uninterrupted with PVE3.2 (4VM's, 2 kvm Win...